Revert "ANDROID: Don't add image to all target with KBUILD_MIXED_TREE"

This reverts commit 0d018732.

Reason for revert: With Kleaf, MAKE_GOALS is always specified. Unless one uses kernel_build(make_goals = ["all"]), this case is never hit.
In that case, they should just drop "all" and use "modules" instead.

Bug: 402231876
Change-Id: I9f85c839c4e33e8a232190b5143ef3453eac1850
Signed-off-by: HONG Yifan <elsk@google.com>

arch/arm64/Makefile

@@ -174,10 +174,7 @@ else
 KBUILD_IMAGE	:= $(boot)/vmlinuz.efi
 endif
 
-# Don't compile Image in mixed build with "all" target
-ifndef KBUILD_MIXED_TREE
 all:	$(notdir $(KBUILD_IMAGE))
-endif
 
 image.fit: dtbs
 

arch/x86/Makefile

@@ -299,11 +299,8 @@ BOOT_TARGETS = bzdisk fdimage fdimage144 fdimage288 hdimage isoimage
 
 PHONY += bzImage $(BOOT_TARGETS)
 
-# Don't compile Image in mixed build with "all" target
-ifndef KBUILD_MIXED_TREE
 # Default kernel to build
 all: bzImage
-endif
 
 # KBUILD_IMAGE specify target image being built
 KBUILD_IMAGE := $(boot)/bzImage