ST21i Removed boot flag I was trying to boot from the sdcard, as I thought it was a boot order issue, I disabled the boot flag on the boot partition. Now my phone will not turn on, or enter bootloader. I have found various guides to booting from an sdcard, however they require flipping a switch on the board, which does not appear to be there. Any ideas on how to fix this situation. I would like to be able to get the phone to a working state again, without having to wipe it.

Edit: Having looked a bit more, the phone is detected by the kernel, as it displays in dmesg and is also listed by lsusb. This is on ArchLinux 5.2.13 I have tried qmicli but that appears to only work if the driver is cdc-mdm whereas it is using qcserial. when connected, it generates /dev/ttyUSB0 and /dev/bus/usb/002/* which is a character unbuffered device file.
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