package/linux-backports: fix kernel version check

The commit 05fea6e4 "infra/pkg-kconfig:
do not rely on package's .config as a timestamp" broke the kernel
version check of this linux-backports package (it was no longer
executed). Since linux-4.19, the kernel's build system internally
touches its .config file, so it can no longer be used as a stamp file.
The stamp file defined in KCONFIG_STAMP_DOTCONFIG variable of
pkg-kconfig infra need to be used instead.

This commit fixes the kernel version check.

Signed-off-by: Julien Olivain <ju.o@free.fr>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Tested-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>