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: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200215063852.8298-2-masahiroy@kernel.org