makedevs: support optional files

makedevs: support optional files

Add the 'F' file type to makedevs, that allows a file to be optional.
With this option, the line is just silently skipped if the file doesn't
exist (or is not a regular file).

This is useful for _PERMISSIONS where the file can be configured out
by package-specific configuration that is not directly handled by
Buildroot, like busybox.

Cc: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Niranjan Reddy <niranjan.reddy@rockwellcollins.com>
Cc: Bryce Ferguson <bryce.ferguson@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>