package/pkg-kconfig: fix reconfigure for kconfig packages

Commit 4b81badb

    Currently, calling foo-reconfigure for a kconfig-based package will not
    re-trigger the configuration (kconfig-wise) step for the package.

was supposed to solve this problem and lately we had

Commit 05fea6e4

    infra/pkg-kconfig: do not rely on package's .config as a timestamp

that introduced the .stamp_dotconfig file.

For this reason, to trigger a kconfig package reconfigure is now
necessary to remove the .stamp_dotconfig file.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>