Fix booting on some arm64 systems after r327879 by fixing the call to
utf8_to_ucs2 in boot1.efi. We need to initialise the ucs2 output string
so it will allocate space, and use the return value to determine if the
call was successful.

Reviewed by: imp
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D13915