brcm47xx: increase max image size

the trx utile uses a maximum image size of 7.2MB. There are brcm47xx
devices even with serial flash with bigger flash chips, but OpenWrt was
not able to create images for these devices. This patch provides an
additional parameter which increases this limit to 32 MB. There is a
warning in the trx utile code which suggests that bigger images could
overwrite the nvram partition on some devices, but normally the program
writing the image should make sure that it is safe to write it to the
flash.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 46872