build: grab common EFI source files in arch specific dir

build: grab common EFI source files in arch specific dir

Rather than preparing the efi source file, we will make the symbolic
link as needed from the build location.

The `ln` command is run every time to allow to update the link in case
the source tree change location.

This patch also introduce "efi-common.mk" which allow to reuse the
common make instructions without having to duplicate them into each
arch.

And now that we have a list of common source file, we can start to
remove the links to the source files on clean.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>