stand: Move eficom to libefi

Rename efiserialc to eficom.c and move it to libefi. Remove
loader.efi.h, since it's not needed. It's architecture independent
(though how we use it might vary). Drivers also belong in libfoo
in the boot loader: all the BIOS drivers are in i386/libi386 and
the console driver is in efi/libefi.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D40007