payloads/tianocore: Fix bootsplash/logo handling

commit 108e537928
("payloads/tianocore: Add a proper target for the Boot Splash")
introduced 2 bugs in bootsplash handling:

- the "logo" make target added a spurious "/edk2" to the project dir
- the "logo" make target failed to account for the case where no user-
  defined logo file is used (the upstream Tianocore one will be used
  in this case)

Fix both these issues.

Test: build/boot qemu w/Tianocore w/o user-defined bootsplash file.

Change-Id: Ieebc547670213459823f58956ae87c6bf94b74ef
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66142
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>

payloads/external/tianocore/Makefile

@@ -136,8 +136,9 @@ update: $(project_dir)
 		fi; \
 		git submodule update --init --checkout
 
-logo: $(project_dir)/edk2
+logo: $(project_dir)
 	case "$(CONFIG_TIANOCORE_BOOTSPLASH_FILE)" in \
+		"") ;; \
 		/*) convert -background None $(CONFIG_TIANOCORE_BOOTSPLASH_FILE) \
 			BMP3:$(project_dir)/MdeModulePkg/Logo/Logo.bmp;; \
 		*) convert -background None $(top)/$(CONFIG_TIANOCORE_BOOTSPLASH_FILE) \