busybox

gen_build_files.sh: rewrite with sed

The shell parsing of files is incredibly slow on many systems.  With
one report, the process was taking a minute or two which made people
thing the build was hung.  So rewrite the craziness with sed and proper
shell functions.  On an idle system, this cut the runtime by half.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>