payloads/tianocore: Fix check for custom bootsplash

-n needs to check against a string, but if CONFIG_TIANOCORE_BOOTSPLASH_FILE
is unset, then $(CONFIG_TIANOCORE_BOOTSPLASH_FILE) evaluates to nothing
and the check fails, leading the Makefile to try and copy a non-
existant file/path.

Change-Id: Iff717dd48748cff16f485bafaa91c7a225fb5bdb
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40860
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

payloads/external/tianocore/Makefile

@@ -94,7 +94,7 @@ checktools:
 build: update checktools
 	unset CC; $(MAKE) -C $(project_dir)/BaseTools
 	echo " build $(project_name) $(TAG)"
-	if [ -n $(CONFIG_TIANOCORE_BOOTSPLASH_FILE) ]; then \
+	if [ -n "$(CONFIG_TIANOCORE_BOOTSPLASH_FILE)" ]; then \
 		echo "    Copying custom bootsplash image"; \
 		case "$(CONFIG_TIANOCORE_BOOTSPLASH_FILE)" in \
 			/*) cp $(CONFIG_TIANOCORE_BOOTSPLASH_FILE) \