busybox

docs: update new-applet-HOWTO.txt

This patch adds some information about the gen_build_files.sh script and how
it allows to keep the Kbuild, Config.in etc. declarations in .c files.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>