loader.efi: fix EFI getchar() for multiple consoles

This fix is ported from illumos (issue #9970), the analysis and initial
implementation was done by John Levon.

See also: https://www.illumos.org/issues/9970

Currently, efi_cons_getchar() will wait for a key. While this seems to make
sense, the implementation of getchar() in common/console.c will loop across
getchar() for all consoles without doing ischar() first.

This means that if we've configured multiple consoles, we can't input into
the serial, as getchar() will be sat waiting for input only from efi_console.c

This patch does implement a bit more generic key buffer to support
translation of input keys, and we use generic efi_readkey() to reduce
duplication from calls from getchar() and poll().