zephyr: Fix usage of CONFIG_MBEDTLS_BUILTIN and ASN1

This commit fixes issue where even if Zephyr provided mbedTLS
module was used, ASN1 decoding procedures have been taken from
MCUboot own version of mbedTLS extracted sources for these functions.

To be able to do that this commit changes config file used with
mbedTLS with the one provided from Zephyr (the generic one), which
allows to select required mbedTLS features via Kconfig options
exposed by Zephyr.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/zephyr/CMakeLists.txt

@@ -25,8 +25,10 @@ assert_exists(TINYCRYPT_SHA512_DIR)
 set(FIAT_DIR "${MCUBOOT_DIR}/ext/fiat")
 assert_exists(FIAT_DIR)
 # Path to mbed-tls' asn1 parser library.
-set(MBEDTLS_ASN1_DIR "${MCUBOOT_DIR}/ext/mbedtls-asn1")
-assert_exists(MBEDTLS_ASN1_DIR)
+if(NOT CONFIG_MBEDTLS_BUILTIN)
+  set(MBEDTLS_ASN1_DIR "${MCUBOOT_DIR}/ext/mbedtls-asn1")
+  assert_exists(MBEDTLS_ASN1_DIR)
+endif()
 set(NRF_DIR "${MCUBOOT_DIR}/ext/nrf")
 
 if(CONFIG_BOOT_USE_NRF_CC310_BL)
@@ -150,14 +152,16 @@ if(CONFIG_BOOT_RAM_LOAD OR CONFIG_SINGLE_APPLICATION_SLOT_RAM_LOAD)
 endif()
 
 if(CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256 OR CONFIG_BOOT_ENCRYPT_EC256)
-  zephyr_library_include_directories(
-    ${MBEDTLS_ASN1_DIR}/include
+  if(MBEDTLS_ASN1_DIR)
+    zephyr_library_include_directories(
+      ${MBEDTLS_ASN1_DIR}/include
     )
-  zephyr_library_sources(
-    # Additionally pull in just the ASN.1 parser from mbedTLS.
-    ${MBEDTLS_ASN1_DIR}/src/asn1parse.c
-    ${MBEDTLS_ASN1_DIR}/src/platform_util.c
+    zephyr_library_sources(
+      # Additionally pull in just the ASN.1 parser from mbedTLS.
+      ${MBEDTLS_ASN1_DIR}/src/asn1parse.c
+      ${MBEDTLS_ASN1_DIR}/src/platform_util.c
     )
+  endif()
   if(CONFIG_BOOT_USE_TINYCRYPT)
   # When using ECDSA signatures, pull in our copy of the tinycrypt library.
   zephyr_library_include_directories(
@@ -207,8 +211,17 @@ elseif(CONFIG_BOOT_SIGNATURE_TYPE_RSA)
   endif()
 elseif(CONFIG_BOOT_SIGNATURE_TYPE_ED25519 OR CONFIG_BOOT_ENCRYPT_X25519)
   if(CONFIG_BOOT_USE_TINYCRYPT)
+    if(MBEDTLS_ASN1_DIR)
+      zephyr_library_include_directories(
+        ${MBEDTLS_ASN1_DIR}/include
+      )
+      zephyr_library_sources(
+        # Additionally pull in just the ASN.1 parser from mbedTLS.
+        ${MBEDTLS_ASN1_DIR}/src/asn1parse.c
+        ${MBEDTLS_ASN1_DIR}/src/platform_util.c
+      )
+    endif()
     zephyr_library_include_directories(
-      ${MBEDTLS_ASN1_DIR}/include
       ${BOOT_DIR}/zephyr/include
       ${TINYCRYPT_DIR}/include
       ${TINYCRYPT_SHA512_DIR}/include
@@ -217,9 +230,6 @@ elseif(CONFIG_BOOT_SIGNATURE_TYPE_ED25519 OR CONFIG_BOOT_ENCRYPT_X25519)
       ${TINYCRYPT_DIR}/source/sha256.c
       ${TINYCRYPT_DIR}/source/utils.c
       ${TINYCRYPT_SHA512_DIR}/source/sha512.c
-      # Additionally pull in just the ASN.1 parser from mbedTLS.
-      ${MBEDTLS_ASN1_DIR}/src/asn1parse.c
-      ${MBEDTLS_ASN1_DIR}/src/platform_util.c
       )
     zephyr_library_compile_definitions(
       MBEDTLS_CONFIG_FILE="${CMAKE_CURRENT_LIST_DIR}/include/mcuboot-mbedtls-cfg.h"

boot/zephyr/Kconfig

@@ -131,6 +131,8 @@ config BOOT_SIGNATURE_TYPE_RSA
 	bool "RSA signatures"
 	select BOOT_USE_MBEDTLS
 	select MBEDTLS
+	select MBEDTLS_ASN1_PARSE_C if MBEDTLS_BUILTIN
+	select MBEDTLS_KEY_EXCHANGE_RSA_ENABLED if MBEDTLS_BUILTIN
 	select BOOT_ENCRYPTION_SUPPORT
 	select BOOT_IMG_HASH_ALG_SHA256_ALLOW
 
@@ -181,6 +183,8 @@ config BOOT_ED25519_MBEDTLS
 	bool "Use mbedTLS"
 	select BOOT_USE_MBEDTLS
 	select MBEDTLS
+	select MBEDTLS_ASN1_PARSE_C if MBEDTLS_BUILTIN
+
 endchoice
 endif
 
@@ -225,8 +229,13 @@ config MCUBOOT_CLEANUP_RAM
 	help
 	  Sets contents of memory to 0 before jumping to application.
 
+if MBEDTLS
+
 config MBEDTLS_CFG_FILE
-	default "mcuboot-mbedtls-cfg.h"
+	default "config-tls-generic.h" if MBEDTLS_BUILTIN
+	default "mcuboot-mbedtls-cfg.h" if BOOT_USE_MBEDTLS
+
+endif
 
 config BOOT_HW_KEY
 	bool "Use HW key for image verification"