buildsystem: add CONFIG_SECCOMP

buildsystem: add CONFIG_SECCOMP

Until now, this feature was switched on via the kernel configuration
option KERNEL_SECCOMP.

The follwing change a7f794cd2aa104fdbd4c6e38f9b76373bf9b96e1 now requires that
the package procd-seccomp must also enabled for buildinmg.

However, this is not the case we have no dependency and the imagebuilder
cannot build the image, because of the implicit package selection.

This change adds a new configuration option CONFIG_SECCOMP.
The new option  has the same behaviour as the configuration
option CONFIG_SELINUX.

If the CONFIG_SECCOMP is selected then the package procd-seccomp and
KERNEL_SECCOMP is enabled for this build.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>