Fix makefile include order by using sort/wildcard.

The 'include' directive in GNU make supports wildcards, but their
expansion has no defined sort order (GLOB_NOSORT is passed to glob()).
Usually this doesn't matter. However, there is at least one case where
it does make a difference: toolchain/*/*.mk includes both the
definitions of the external toolchain packages and
pkg-toolchain-external.mk, but pkg-toolchain-external.mk must be
included first.

For predictability, use ordered 'include $(sort $(wildcard ...))'
instead of unordered direct 'include */*.mk' everywhere.

Fixes [1] reported by Petr Vorel:

  make: *** No rule to make target 'toolchain-external-custom', needed by '.../build/toolchain-external/.stamp_configured'.  Stop.

[1] http://lists.busybox.net/pipermail/buildroot/2017-November/206969.html



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Tested-by: Petr Vorel <petr.vorel@gmail.com>
[Arnout: also sort the one remaining include, of the external docs]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>