efi_loader: capsule: add back efi_get_public_key_data()

The commit 47a25e81 ("Revert "efi_capsule: Move signature from DTB to
.rodata"") failed to revert the removal of efi_get_public_key_data().

Add back this function and move it under lib/efi_loader so that other
platforms can utilize it. It is now declared as a weak function so that
it can be replaced with a platform-specific implementation.

Fixes: 47a25e81 ("Revert "efi_capsule: Move signature from DTB to
	.rodata"")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>