fix umount error when build is done

fix umount error when build is done

error: there're mounted directories to build root. Please unmount them manually to avoid being deleted unexpectly:
    / ==> ~/GBS-ROOT-TIZEN/t65std/local/BUILD-ROOTS/scratch.armv7l.0/proc
    / ==> ~/GBS-ROOT-TIZEN/t65std/local/BUILD-ROOTS/scratch.armv7l.0/proc/sys/fs/binfmt_misc
    / ==> ~/GBS-ROOT-TIZEN/t65std/local/BUILD-ROOTS/scratch.armv7l.0/proc

In certain build environments, proc/fs/sys/fs/binfmt_misc and proc are mounted multiple times (unknown reason)
However, when the build is complete, umount is executed only once.

So, I added the repeating code to umount them all.

Change-Id: Ia8cdfc62d0481043aae2b951a6daa202ba4b4a08
Signed-off-by: Hyunggi Lee <hyunggi.lee@samsung.com>