powerpc: make the install target not depend on any build artifact

The install target should not depend on any build artifact.

The reason is explained in commit 19514fc6 ("arm, kbuild: make
"make install" not depend on vmlinux").

Change the PowerPC installation code in a similar way.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210729141937.445051-2-masahiroy@kernel.org

arch/powerpc/boot/Makefile

@@ -445,7 +445,7 @@ $(obj)/zImage.initrd:	$(addprefix $(obj)/, $(initrd-y))
 	$(Q)rm -f $@; ln $< $@
 
 # Only install the vmlinux
-install: $(CONFIGURE) $(addprefix $(obj)/, $(image-y))
+install:
 	sh -x $(srctree)/$(src)/install.sh "$(KERNELRELEASE)" vmlinux System.map "$(INSTALL_PATH)"
 
 PHONY += install

arch/powerpc/boot/install.sh

@@ -20,6 +20,20 @@
 # Bail with error code if anything goes wrong
 set -e
 
+verify () {
+	if [ ! -f "$1" ]; then
+		echo ""                                                   1>&2
+		echo " *** Missing file: $1"                              1>&2
+		echo ' *** You need to run "make" before "make install".' 1>&2
+		echo ""                                                   1>&2
+		exit 1
+	fi
+}
+
+# Make sure the files actually exist
+verify "$2"
+verify "$3"
+
 # User may have a custom install script
 
 if [ -x ~/bin/${INSTALLKERNEL} ]; then exec ~/bin/${INSTALLKERNEL} "$@"; fi