ipq40xx: Linksys: sysupgrade: Ensure OEM volumes are removed

ipq40xx: Linksys: sysupgrade: Ensure OEM volumes are removed

When OEM volumes are present in the [alt_]firmware partition,
sysupgrade will write a new kernel, but will fail to write
the root file system. The next boot will hang indefinitely

    Waiting for root device /dev/ubiblock0_0...

Modified ipq40xx/base-files/lib/upgrade/linksys.sh
to remove both `squashfs` and `ubifs` if found
on the target firmware partition's UBI device.

Run-tested-on: Linksys EA8300

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[applied some shellcheck suggestions as well]