roms: seabios: Rename CROSS_COMPILE to CROSS_PREFIX

roms: seabios: Rename CROSS_COMPILE to CROSS_PREFIX

SeaBIOS introduced CROSS_PREFIX in 2013 but it's not set in roms
Makefile.

With the change it's possible to cross-compile SeaBIOS on macOS,
if acpica/iasl is installed:
  cd roms
  export PATH=/path/to/cross/x86_64-unknown-linux-gnu/bin:$PATH
  make bios system=unknown-linux-gnu

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-id: 20181121203720.75916-1-r.bolshakov@yadro.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>