package/linux: handle generated defconfigs

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
[yann.morin.1998@free.fr: move the kconfig-package hunk to the
 corresponding patch]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>