Updated release management of ble central firwares

Now only latest f/w will be kept in release folder.

	new file:   README.txt
	renamed:    v3/GeckoCentral_v2_256k.hex -> SL_V4.0_Central_01.00.00.hex
	deleted:    v2/GeckoCentral_v2_DEBUG.hex
	deleted:    v2/README.txt
	deleted:    v3/README.txt