RRO: idmap: allow flock(2) to block

During creation of an idmap an advisory lock is applied on the
output file using flock(2). This commit removes the LOCK_NB flag
from the call to flock(2).

The reason for this is that if the function were to return
EWOULDBLOCK, the entire idmap operation would be aborted leading
to the device booting without the correct overlay packages applied.

Change-Id: Iad319779976e950d4354e4f60b30439f6f208b77