tegra-storage-layout-base: fix typo on external layout

Fix typo and missing 'T' character in variable reference
for `PARTITION_LAYOUT_EXTERNAL`.

The current typo means if the name of the file referenced
by PARTITION_FILE_EXTERNAL was different than the name
PARTITION_LAYOUT_EXTERNAL, the wrong filename would be used
in the l4t-storage-layout dir.

Relates to #1

1: #1684

Signed-off-by: Dan Walkes <danwalkes@trellis-logic.com>

recipes-bsp/tegra-binaries/tegra-storage-layout-base_35.5.0.bb

@@ -10,7 +10,7 @@ PARTITION_FILE_EXTERNAL ?= "${S}/tools/kernel_flash/${PARTITION_LAYOUT_EXTERNAL}
 do_install() {
     install -d ${D}${datadir}/l4t-storage-layout
     install -m 0644 ${PARTITION_FILE} ${D}${datadir}/l4t-storage-layout/${PARTITION_LAYOUT_TEMPLATE}
-    [ -z "${PARTITION_LAYOUT_EXTERNAL}" ] || install -m 0644 ${PARTITION_FILE_EXTERNAL} ${D}${datadir}/l4t-storage-layout/${PARTITION_LAYOUT_EXERNAL}
+    [ -z "${PARTITION_LAYOUT_EXTERNAL}" ] || install -m 0644 ${PARTITION_FILE_EXTERNAL} ${D}${datadir}/l4t-storage-layout/${PARTITION_LAYOUT_EXTERNAL}
 }
 
 PACKAGES = "${PN}-dev"