fs_mgr: Allow commas in boot device paths.

If the value of androidboot.boot_devices has a comma in its path, it'll
get split and /dev/block/by-name symlinks won't work. As a workaround,
add "androidboot.boot_device" as a valid command-line parameter. It can
be repeated multiple times to retain multiple-boot-device semantics.

Bug: 153024538
Test: device boots with androidboot.boot_device= set
Change-Id: I1be5aeec287ba198768c3452b1930d59b2f13463