bcm53xx: fix UBI EOF patch to allow sysupgrade after first installation

bcm53xx: fix UBI EOF patch to allow sysupgrade after first installation

With previous version of patch info about need of erasing blocks was
stored once per boot. It was breaking in following scenario:
1) First boot after installation (erasing blocks after 0xdeadc0de)
2) Doing sysupgrade (with ubidetach & ubiformat)
3) Attaching UBI again (it caused all blocks to be erased)

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45387 3c298f89-4303-0410-b956-a3cf2f4a3e73