boot/barebox: check for config file before calling kconfig-package

If we check that the user provides a config file after we call to the
kconfig-package infra, the error message we get is the one for the
kconfig-package infra, not the custom error message we want to show to
the user.

So, only call kconfig-package after we do the check.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>