brcm47xx: check for .ELF header in partition parser

The image format for the wgt634u is different from the normal format. 
Without this patch the partition parser detects a 128kb bigger 
partition after every update.

This closes #11420

Thank you b.sander for the patch.


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33935 3c298f89-4303-0410-b956-a3cf2f4a3e73