populate_sdk_ext.bclass: make sure OECORE_NATIVE_SYSROOT is exported.

Fixes bug 15464.

OECORE_NATIVE_SYSROOT is correctly set up and exported in the SDK's
environment file. But it's then unset in buildtools/environment-setup-*.
The value is restored in the SDK's environment file but is not exported
again.

(From OE-Core rev: 825c996b7995d3ad510933b1a88229831ca5ea29)

Signed-off-by: Gauthier HADERER <ghaderer@wyplay.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/populate_sdk_ext.bbclass

@@ -724,7 +724,7 @@ sdk_ext_postinst() {
 		echo "# Save and reset OECORE_NATIVE_SYSROOT as buildtools may change it" >> $env_setup_script
 		echo "SAVED=\"\$OECORE_NATIVE_SYSROOT\"" >> $env_setup_script
 		echo ". $target_sdk_dir/buildtools/environment-setup*" >> $env_setup_script
-		echo "OECORE_NATIVE_SYSROOT=\"\$SAVED\"" >> $env_setup_script
+		echo "export OECORE_NATIVE_SYSROOT=\"\$SAVED\"" >> $env_setup_script
 	fi
 
 	# Allow bitbake environment setup to be ran as part of this sdk.