brcm47xx: revert r33935: check for .ELF header in partition parser

brcm47xx: revert r33935: check for .ELF header in partition parser

For sysupgrade the generic image should be used and the special image for the wgt634u is just needed for initial flashing over CFE.
russell reported that with this patch sysupgrdae did not worked for him any more and he could not reproduce the error described in #11420.
With the patch the linux partition also contained the loader which is in the first 128KB.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34201 3c298f89-4303-0410-b956-a3cf2f4a3e73