When the EFI implementation (like U-Boot) does not support WaitForKey, we
can emulate efi_cons_poll(0 with a flag and caching the last key read with
ReadKeyStroke. This fixes the loader.efi countdown timer on Pine64 (and
other U-Boot + EFI using platforms).

Reviewed by: imp, manu
Differential Revision: https://reviews.freebsd.org/D7670