leon3: fix the error message when no bios are provided

leon3: fix the error message when no bios are provided

The leon3 board is looking for u-boot.bin by default (LEON3_PROM_FILENAME)..
But in the case this file is not found and no other file are given on the
command line we get the following error:

  $ ./qemu-system-sparc -M leon3_generic
  qemu-system-sparc: Can't read bios image (null)

So use LEON3_PROM_FILENAME instead of filename in case it is NULL to get a
less cryptic message:

  $ ./qemu-system-sparc -M leon3_generic
  qemu-system-sparc: Can't read bios image 'u-boot.bin'

Suggested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>