[doc]: update build instruction for debian stretch

Signed-off-by: Guohan Lu <gulv@microsoft.com>

README.md

@@ -62,7 +62,7 @@ To build SONiC installer image and docker images, run the following commands:
     BLDENV=stretch make stretch
 
     # build SONiC image
-    make
+    make all
 
  **NOTE**:
 
@@ -83,6 +83,14 @@ The SONiC installer contains all docker images needed. SONiC uses one image for
 
 For Broadcom ASIC, we build ONIE and EOS image. EOS image is used for Arista devices, ONIE image is used for all other Broadcom ASIC based devices. 
 
+    make configure PLATFORM=broadcom
+    # build debian stretch required targets
+    BLDENV=stretch make stretch
+    # build ONIE image
+    make target/sonic-broadcom.bin
+    # build EOS image
+    make target/sonic-aboot-broadcom.swi
+
 You may find the rules/config file useful. It contains configuration options for the build process, like adding more verbosity or showing dependencies, username and password for base image etc.
 
 Every docker image is built and saved to target/ directory.