Revert "Use stamp file rather than grouped targets for TF-A."

This reverts commit 17a8b9700b06e885e5eba17215c7e2103e61b401.

Reason for revert: With https://android-kvm-review.googlesource.com/c/prebuilts/+/2640 and related changes we now use a more recent version of make and can use grouped targets.

Change-Id: I23c839a8590de3e7ce536721cf5561dfb82b1cef

Makefile.root

diff --git a/Makefile.root b/Makefile.root
index 72fba1b..48cb146 100644
--- a/Makefile.root
+++ b/Makefile.root
@@ -211,7 +211,6 @@
 ##
 TFA_SRC := $(ROOT_DIR)/trusted-firmware-a
 TFA_OUT := $(OUT_DIR)/trusted-firmware-a
-TFA_STAMP := $(TFA_OUT)/trusted-firmware-a.stamp
 TFA_TOOLCHAIN ?= gcc-9.2
 TFA_VERBOSE ?= $(VERBOSE)
 $(eval $(call define_toolchain,$(TFA_TOOLCHAIN),TFA_))
@@ -232,11 +231,8 @@
 .PHONY: tfa
 tfa: $(TFA_OUT)/qemu/release/bl1.bin $(TFA_OUT)/qemu/release/bl2.bin $(TFA_OUT)/qemu/release/bl31.bin
 
-# This should use grouped targets, but we don't have make 4.3 in our CI.
-$(TFA_OUT)/qemu/release/bl1.bin $(TFA_OUT)/qemu/release/bl2.bin $(TFA_OUT)/qemu/release/bl31.bin: $(TFA_STAMP)
-$(TFA_STAMP): FORCE
+$(TFA_OUT)/qemu/release/bl1.bin $(TFA_OUT)/qemu/release/bl2.bin $(TFA_OUT)/qemu/release/bl31.bin &: FORCE
 	+ $(TFA_MAKE) all
-	touch $(TFA_STAMP)
 
 .PHONY: tfa_clean
 tfa_clean: