payloads/Memtest86Plus: Update branch name from master to main

It seems like the default branch for coreboot's Memtest86+ fork was
renamed from 'master' to 'main'.

TEST: Memtest builds correctly when selecting 'Main' for the Memtest86+
version option.

Change-Id: I269249518019f5d0d12c57f1c14012abca86b48b
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76100
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/external/Memtest86Plus/Makefile

@@ -1,7 +1,7 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
-TAG-$(CONFIG_MEMTEST_MASTER)=origin/master
-NAME-$(CONFIG_MEMTEST_MASTER)=Master
+TAG-$(CONFIG_MEMTEST_MAIN)=origin/main
+NAME-$(CONFIG_MEMTEST_MAIN)=Main
 TAG-$(CONFIG_MEMTEST_STABLE)=0bd34c22604660e4283316331f3e7bf8a3863753
 NAME-$(CONFIG_MEMTEST_STABLE)=Stable
 TAG-$(CONFIG_MEMTEST_REVISION)=$(CONFIG_MEMTEST_REVISION_ID)
@@ -26,14 +26,14 @@ endif
 	false
 endif
 	-cd $(project_dir); git show $(TAG-y) >/dev/null 2>&1 ; \
-		if [ $$? -ne 0 ] || [ "$(TAG-y)" = "origin/master" ]; then \
+		if [ $$? -ne 0 ] || [ "$(TAG-y)" = "origin/main" ]; then \
 		echo "    Fetching new commits from the $(project_name) git repo"; \
 		git fetch; fi
 
 checkout: fetch
 	echo "    Checking out $(project_name) revision $(NAME-y) ($(TAG-y))"
 	cd $(project_dir); \
-		git checkout master; \
+		git checkout main; \
 		git branch -D coreboot 2>/dev/null; \
 		git checkout -b coreboot $(TAG-y)