infra/pkg-kconfig: don't require a kconfig file for disabled packages

When a package is not enabled, we need not check that it provides
a .config/defconfig file.

So far, all our pkg-kconfig packages unconditionally set their
_KCONFIG_FILE, even to an empty value. But some packages (e.g.
at91bootstrap3) wanted to set it only when enabled, and broke.

So, only do the check when the package is enabled.

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