In order to relock the bootloader you must firdt change back to a stock kernel, otherwise you will brick the phone, wich luckily is recoverable...
Even if yo relock on custom kernel, you will also can enter in fastboot mode to flash stock one
Sent from my little big badass boy!
Sent from Quo'nos via subspace transmission.
SonyEricsson Xperia X8, Neo, Arc, SGSII, SGSIII, Sony Xperia Z (LTE).