efi/libstub: consolidate initrd handling across architectures

Before adding TPM measurement of the initrd contents, refactor the
initrd handling slightly to be more self-contained and consistent.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://lore.kernel.org/r/20211119114745.1560453-4-ilias.apalodimas@linaro.org


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>