mtd: fix static build with uuid

The uuid library might depend on gettext. Use pkg-config to discover uuid
dependencies at build time, and edit Makefile accordingly to avoid missing
symbols when linking statically.

Based on a patch from Vicente Olivert Riera.

Fixes:
http://autobuild.buildroot.net/results/2dd/2ddd714446d9eb75701bd48c117dc5bbbd291a76/
http://autobuild.buildroot.net/results/3af/3af4ef252c8cb2b22134d91ce48e5930c60ec437/
http://autobuild.buildroot.net/results/648/648341fce974e8f016eee57c95d7511a886eef49/



[Peter: drop MTD_MKFSUBIFS_LIBS variable]
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>