payloads/tianocore: Fix unclean working directory detection

After commit ae48b42683 (payloads/tianocore: Init submodules),
Tianocore's Makefile no longer detects an unclean working directory and
thus always performs a `git checkout`, overwriting any uncommited
changes made in the cloned sources.

The change of "clean" to "dirty" effectively inverts the logic of the
if-else condition, which would normally swap the two possible code paths
of the branch. However, since `git status` outputs multiple lines, most
of which do not contain "clean", the -v option (select non-matching
lines) causes grep to always match at least 1 line and thus return
success.  This causes the if-else branch containing the `git checkout`
to always be taken regardless of the state of the working tree, masking
the issue of the inverted logic.  Removing the -v option addresses both
of these issues and restores the intended behavior of the if-else block.

TEST:
1) Build coreboot successfully with the Tianocore UefiPayloadPkg option.
2) Make a change in the cloned Tianocore sources that results in an
   unclean working directory and check for the "Working directory not
clean" message when building coreboot.

Change-Id: Icd4952b40c147d0fba676089ced5a8b59b93ad50
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64608
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@tutanota.com>

payloads/external/tianocore/Makefile

@@ -100,7 +100,7 @@ update: $(project_dir)
 			echo "    $(CONFIG_TIANOCORE_TAG_OR_REV) is not a valid git reference"; \
 			exit 1; \
 		fi; \
-		if git status --ignore-submodules=dirty | grep -qv clean; then \
+		if git status --ignore-submodules=dirty | grep -q clean; then \
 			echo "    Checking out $(project_name) revision $(CONFIG_TIANOCORE_TAG_OR_REV)"; \
 			git checkout --detach $(CONFIG_TIANOCORE_TAG_OR_REV) -f; \
 		else \