rtcd.pl: Simplify parse of aom_config.h.
rtcd.pl: Simplify parse of aom_config.h.

Also updates help text, and addresses DSPR2/MSA RTCD support
error introduced in 7f140894.

Change-Id: I9c1335d4245846955907f74db38e0fbc01e8d641

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom