config: rename SECCOMP to USE_SECCOMP

config: rename SECCOMP to USE_SECCOMP

It seems that we have some kind of a symbol name conflict which causes
CONFIG_SECCOMP to always be read as y.

Unfortunatelly, I could not figure out what is causing this, but simply
renaming SECCOMP to USE_SECCOMP seems to properly work and leaves the
symbol unset unless arch dependencies are satisfied.

This fixes qoriq and others that dont support seccomp from failing due
to procd-seccomp package being selected to get included but it cannot be
built for them:
ERROR: unable to select packages:
  procd-seccomp (no such package):
    required by: base-files-1637~52b6c92479[procd-seccomp]

Fixes: 4c65359af49b ("build: fix including busybox, procd and apk/opkg in imagebuilder")
Link: https://github.com/openwrt/openwrt/pull/17048
Signed-off-by: Robert Marko <robimarko@gmail.com>