kbuild: efi: Avoid rebuilding efi targets

Add a couple of missing targets so that helloworld and other efi targets
are not needlessly rebuilt.

CC: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>