buildroot

infra/pkg-kconfig: commonalise update-(def)config code

Except for the nice human-friendly reminder of the command that the user
was just running, and the name of the file to copy from, those two rules
are exactly the same.

Make that a common macro that is shared, so that it's easier to add more
checks, and to simplify maintenance.

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