populate_sdk_ext: Add variable to indicate running in eSDK

This allows for other scripts to know that they are being executed in
the context of the eSDK in order to provide different behaviour as
needed.

[YOCTO #11155]

(From OE-Core rev: ebdd9b5a19eb6fb6c3249be4f0e2480a3de2aa8f)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/populate_sdk_ext.bbclass

@@ -351,6 +351,9 @@ python copy_buildsystem () {
             # Hide the config information from bitbake output (since it's fixed within the SDK)
             f.write('BUILDCFG_HEADER = ""\n\n')
 
+            f.write('# Provide a flag to indicate we are in the EXT_SDK Context\n')
+            f.write('WITHIN_EXT_SDK = "1"\n\n')
+
             # Map gcc-dependent uninative sstate cache for installer usage
             f.write('SSTATE_MIRRORS += " file://universal/(.*) file://universal-4.9/\\1 file://universal-4.9/(.*) file://universal-4.8/\\1"\n\n')