[build] Update `update_mbed_repo` target
[fix][build] Revision variable name
[fix][mbed] Add missing `common.h`
[fix][mbed] Remove stale assertions
[fix][mbed] Remove stale `send_all()`
[fix][mbed] Add missing declarations
[mbed] Provide default setting for `XI_IO_LAYER` -

user may override this in `xi_user_config.h`, but they must know
what they are doing then!