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>

package/makedevs/makedevs.c

@@ -570,9 +570,12 @@ int main(int argc, char **argv)
 				ret = EXIT_FAILURE;
 				goto loop;
 			}
-		} else if (type == 'f') {
+		} else if (type == 'f' || type == 'F') {
 			struct stat st;
 			if ((stat(full_name, &st) < 0 || !S_ISREG(st.st_mode))) {
+				if (type == 'F') {
+					continue; /*Ignore optional files*/
+				}
 				bb_perror_msg("line %d: regular file '%s' does not exist", linenum, full_name);
 				ret = EXIT_FAILURE;
 				goto loop;