boot: zephyr: remove [EXPERIMENTAL] from config option
boot: zephyr: remove [EXPERIMENTAL] from config option

Swap without scratch has been tested enough and no obvious bugs (or
"show stopper) bugs seem to exist; so remove [EXPERIMENTAL] and make
it "stable".

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,utzig/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot