tools/firmware-utils: Allow changing the CRC32 poly value in mkbrnimg

VGV7519 is currently the only device with brnboot support. It seems to
be happy with 0x2083b8ed as CRC32 poly. However, VGV7510KW22 fails to
validate the checksum - it requires 0x04c11db7 instead.

I have built an brnboot image manually on the command line, once with
the old code and then with the new code but passing the old CRC32
poly value. Both resulted in a brnboot image with the same sha1sum.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45881 3c298f89-4303-0410-b956-a3cf2f4a3e73