omb_main: umount OMB_MAIN_DIR to prevent data loss

Keeping OMB_MAIN_DIR mounted can cause data loss, so we need to umount OMB_MAIN_DIR

Additionally dmesg complains about unchecked fs:
EXT3-fs (sda1): warning: mounting unchecked fs, running e2fsck is recommended
Merge pull request #3 from athoik/fixreboot

omb_main: umount OMB_MAIN_DIR to prevent data loss