Ignore error on existing directory

- Now that we install firmware-atheros, /lib/firmware exists in the
chroot. Although that means we don't strictly need this mkdir at all,
leave it in and run it with -p to ignore the error if it exists, just in
case.

Change-Id: Ib480077e30cb0d96bb4a1fb00af9c7bc550c3fb5
1 file changed