mkfwimage: Increase RSPRO maximum size to 0xF00000 (15 MiB)

RSPRO has 16MiB of Flash, using the following layout:

[    2.930000] 0x000000000000-0x000000030000 : "RedBoot"
[    2.940000] 0x000000030000-0x000000130000 : "kernel"
[    2.950000] 0x000000130000-0x000000ff0000 : "rootfs"
[    2.960000] 0x000000ff0000-0x000000fff000 : "FIS directory"
[    2.970000] 0x000000fff000-0x000001000000 : "RedBoot config"

Firmware images contain kernel and rootfs, so they can use up to 0xFF0000 -
0x30000 = 0xFC0000 (15,75MiB) of space. We don't include the RedBoot and FIS
partitions and leave an extra 0xC0000 (768KiB) of space.

This is based on revision 18410 which sets the limit to 0xB00000 (11 MiB).

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38484