utils/genrandconfig: check core-dependencies

When generating a random configuration, also check the host
dependencies for that configuration and error out if it fails.

This is mainly intended for the autobuilders, so they don't try to
build a configuration for which the necessary host packages are not
installed.

Instead of erroring out, we could also generate a different random
config. However:

- When called for the autobuilders, the autobuild-run script will
  anyway just attempt another build. The only difference is that a
  warning will be printed in this autobuild log now. This warning
  is in fact quite useful to hint that the autobuilder could benefit
  from intalling this additional host package.

- When called manually, the user may in fact want to install the
  missing host package and continue with the saved random config.

Fixes:

http://autobuild.buildroot.net/results/d7c/d7c12e873a71fef1e0b5e19cfad45024050614e1
http://autobuild.buildroot.net/results/a0a/a0a105c1ec2d0517112b02932a85831d4b597b9f
http://autobuild.buildroot.net/results/96c/96cc6cedc64c777e993671204809599a1498472a



and many others...

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>