uboot: check various configuration parameters and bail out if empty

Following the issues reported by Jerry <g4@novadsp.com>, it appears
that there are a number of U-Boot configuration variables we are not
checking properly, leaving users with weird behavior at build time.

This commit extends the existing U-Boot checks to verify that when a
custom version, custom tarball, or custom Git or Mercurial
repositories are selected, the appropriate fields are not empty in the
configuration.

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