docs: improve flashing device section

[BBBmau]

Was running into issues with ./flash_system.sh until running into this Discord message which led me to finding instructions within ./flash_system.sh

The error:

└─(22:53:12 on fix-system-build-errors ✹)──> ./flash_system.sh            
Checking active slot...
Traceback (most recent call last):
  File "/Volumes/agnos/agnos-builder/tools/edl_repo/edl", line 393, in <module>
    base.run()
  File "/Volumes/agnos/agnos-builder/tools/edl_repo/edl", line 290, in run
    conninfo = self.doconnect(loop)
  File "/Volumes/agnos/agnos-builder/tools/edl_repo/edl", line 200, in doconnect
    self.cdc.connected = self.cdc.connect(portname=self.portname)
  File "/Volumes/agnos/agnos-builder/tools/edl_repo/edlclient/Library/Connection/usblib.py", line 221, in connect
    devices = usb.core.find(find_all=True, backend=self.backend)
  File "/Volumes/agnos/agnos-builder/tools/edl_repo/venv/lib/python3.10/site-packages/usb/core.py", line 1309, in find
    raise NoBackendError('No backend available')
usb.core.NoBackendError: No backend available
Invalid active slot: ''

it would be nicer to just have it be included in the README.md as part of the flash section noting the necessary steps if attempting to flash with M-Series macs

Also mentioned the need to set the device in QDL mode in order to run ./flash_system.sh with no issues.