build system: fix parallel building issue

build system: fix parallel building issue

The files generated by the include/config/MARKER target are in the
dependency list for applets/applet_tables.
If applets/applet_tables is created first during applets_dir then it will
be created again later as part of $(busybox-dirs).
As a result include/applet_tables.h is created again. This time while other
build commands may need it.

Let applets_dir depend on include/config/MARKER to avoid this particular
race condition and create the header files atomically to ensure that the
compiler never sees incomplete files.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>