util-linux: libblkid needs fork(), disable on !MMU

util-linux: libblkid needs fork(), disable on !MMU

The libblkid library in util-linux uses fork. While it seems
potentially possible to disable the parts of the library that use fork
(the only parts using fork are parts related to LVM and Device Mapper,
which are said to be legacy as they are replaced by sysfs based
scanning, instead of having to fork to run a separate process).

However, since libblkid is used by:

 - e2fsprogs, which uses lots of fork all over the place
 - udev, which most likely will also need fork

There is not much point in making libblkid fork-less. As a
consequence, this commit makes util-linux/libblkid unavailable on
non-MMU architectures, and propagates the relevant dependency to the
reverse dependencies of libblkid.

This fixes the e2fsprogs build failure seen on Blackfin. The failure
was due to a configure test of e2fsprogs which was trying to link a
small test program against libblkid, which failed because there was an
undefined reference to fork() in the libblkid code.

Fixes:

  http://autobuild.buildroot.net/results/ee2c1568d16ac040011dd4d6d8b543ff9e9e2622/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>