To clarify about bootloader unlock/relock :
Q : Can Flashtool unlock bootloader for a first time unlock ?
A : Yes, it does it using official sony method (get key, then issue the right fastboot command)
Q : Can Flashtool relock bootloader ?
A : Yes
Q : Can Flashtool unlock bootloader after a relock ?
A : Yes.
With this release, before unlocking first time, root your device and you will be able to backup your TA partition (Raw ta backup). By doing this, you will be able to relock / restore your TA partition and have back BE2 and all DRM keys.
Flashtool can be downloaded from this page
For those having issues downloading, you can try this link : unrestrict.li Flashtool homepage