efi: stub: use random seed from EFI variable

EFI has a rather unique benefit that it has access to some limited
non-volatile storage, where the kernel can store a random seed. Read
that seed in EFISTUB and concatenate it with other seeds we wind up
passing onward to the kernel in the configuration table. This is
complementary to the current other two sources - previous bootloaders,
and the EFI RNG protocol.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
[ardb: check for non-NULL RNG protocol pointer, call GetVariable()
       without buffer first to obtain the size]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>