x86/boot/build: Add phony targets in arch/x86/boot/Makefile to PHONY

These targets are correctly added to PHONY in arch/x86/Makefile, but
not in arch/x86/boot/Makefile. Thus, with a file 'install' in the top
directory, 'make install' does nothing:

  $ touch install
  $ make install
  make[1]: 'install' is up to date.

Add them to the PHONY targets in the boot Makefile too.

 [ bp: Massage. ]
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200215063852.8298-2-masahiroy@kernel.org

arch/x86/boot/Makefile

@@ -128,6 +128,8 @@ quiet_cmd_genimage = GENIMAGE $3
 cmd_genimage = sh $(srctree)/$(src)/genimage.sh $2 $3 $(obj)/bzImage \
 			$(obj)/mtools.conf '$(image_cmdline)' $(FDINITRD)
 
+PHONY += bzdisk fdimage fdimage144 fdimage288 isoimage bzlilo install
+
 # This requires write access to /dev/fd0
 bzdisk: $(obj)/bzImage $(obj)/mtools.conf
 	$(call cmd,genimage,bzdisk,/dev/fd0)