ST21i Removed boot flag
For booting from MicroSD card you need to heavily modify your ROM. The android boot partition is not like a PC BIOS. You need the bootloader to look at mmcblk1 instead of mmcblk0 which is hard-coded in various system files.
(for more information refer to Irfan Latif's documentation for QMobile Z8 on forum.xda-developers.com)  

If your phone does not boot into fastboot mode it is hard bricked. Most devices have a emergency download mode from where you can repair your bootloader and flash ROM. For Qualcomm devices it is the EDL mode which can be reached via EDL mode cable/dongle or with a testpoint on the PCB circuit board.  

For your old Xperia device you can use Androxyde Flashtool  

For other Qualcomm devices use QPST  

For MediaTek devices use SP Flash Tool