This adds basic file transfer support over BLE.

The central provides a filename and then can transfer chunks of data
which are written to the file.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

By default the MTU is set to 23 bytes, use 241 bytes instead to make the
UART and file transfer service more efficient. The MTU also contains
some BLE headers and is not only the usable data.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

Signed-off-by: Rahix <rahix@rahix.de>