infra/pkg-kconfig: require an non-empty KCONFIG_FILE

Currently, we only check that the variable is defined, which is not
enough since we really want it to be non-empty.

We however can't check it points to an existing file, because the
package might well not be extracted yet, and we may use an internal
defconfig. If that file does not eventually exist, there will be a
failure down the road at build time when we try to copy it...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>