[BUG: #252] rootfs/etc/init.d: fix adding crom entries for system clock monitor

Reduce the possible range in Kconfig to the half of the possible range.
E.g. setting job to be run every 40minutes is not trivial and */40 will
not work. Instead it will run the job at minute 0, then 40, then again
at 0 (after 40min, then after 20min).

Signed-off-by: Adam Wujek <dev_public@wujek.eu>