hw/sparc/leon3: Allow load of uImage firmwares

hw/sparc/leon3: Allow load of uImage firmwares

Currently the Leon3 machine doesn't allow to load legacy u-boot images:

  $ qemu-system-sparc -M leon3_generic -d in_asm \
      -kernel HelenOS-0.6.0-sparc32-leon3.bin
  qemu-system-sparc: could not load kernel 'HelenOS-0.6.0-sparc32-leon3.bin'

  $ file HelenOS-0.6.0-sparc32-leon3.bin
  HelenOS-0.6.0-sparc32-leon3.bin: u-boot legacy uImage, HelenOS-0.6.0,\
    Linux/ARM, OS Kernel Image (Not compressed), 2424229 bytes,\
    Sun Dec 21 19:18:09 2014,\
    Load Address: 0x40000000, Entry Point: 0x40000000,\
    Header CRC: 0x8BCFA236, Data CRC: 0x37AD87DF

Since QEMU can load uImages, add the necessary code,
so the Leon3 machine can load these images:

  $ qemu-system-sparc -M leon3_generic -d in_asm \
      -kernel HelenOS-0.6.0-sparc32-leon3.bin
  ----------------
  IN:
  0x40000000:  b  0x400007a8
  0x40000004:  nop
  ----------------
  IN:
  0x400007a8:  save  %sp, -136, %sp
  0x400007ac:  call  0x40000020
  0x400007b0:  sethi  %hi(0x4000b800), %i1
  ...

Tested with the following firmware:
http://www.helenos.org/releases/HelenOS-0.6.0-sparc32-leon3.bin

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Tested-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-Id: <20190427162922.4207-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>