mtd: fix build when no sub options are enabled

mtd-utils with no sub options enabled should be a noop, but we end up
calling make with the default target (E.G. to build everything), breaking
the build if zlib/lzo aren't available.

Fixes http://autobuild.buildroot.net/results/a0781db394792e6d3831c1884fc4bb3525fedb2e/



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>