Fix umount issue on Ubuntu20.04

Fix umount issue on Ubuntu20.04
When update Ubuntu version to 20.04, After first  gbs build, for some kind reason
During the second gbs build , it will have umount issue:
error: there're mounted directories to build root. Please unmount them manually to avoid being deleted unexpectly:
    / ==> ~/GBS-ROOT/XXX/local/BUILD-ROOTS/scratch.armv7l.0/proc/sys/fs/binfmt_misc
So using umount option -l for lazy umount can fix this issue.
Change-Id: I5f4ea680fefcc38cd9dce30dfa458710c086a399
Signed-off-by: biao716.wang <biao716.wang@samsung.com>