subdir expands the recipe of every target of every build directory when
it creates the rules, although make runs only a few of them. The main
Makefile creates these rules on every make call, and a build calls it
several times in a row.
Put only a call of the recipe into the rules, so that make expands it
when the recipe runs. The commands do not change. The parse of the main
Makefile takes 0.10 s instead of 0.15 s, with 318 package directories.
Signed-off-by: John Crispin <john@phrozen.org>