msp: move constant names to str.[ch].

Signed-off-by: Daniel Stodden <daniel.stodden@gmail.com>
msp: async reponse dispatch tables.

Effectively adding queue depth. Keeps using the kernel output buffer
for async request tx, but multiple requests sent may remain in flight.

Only one request instance per command may be issued. MultiWii has no
transaction identifiers, and therefore the same limitation.

Internally, we maintain a simple call reponse dispatch table. Primary
function is map_call(). Maintains backward-compatible msp_req_send and
msp_rsp_recv implementations, but only for the cli.

Callers provide a return function pointer, a pointer to private state,
and a user-definable timeout. Output data is malloc()ed and left to
the return callee.

Signed-off-by: Daniel Stodden <daniel.stodden@gmail.com>
msp: move sync command implementations to cmd.[ch].

Thereby limiting libmsp.la to the bare minimum amount of code.

Signed-off-by: Daniel Stodden <daniel.stodden@gmail.com>
am: project rename to mwrt.

Signed-off-by: Daniel Stodden <daniel.stodden@gmail.com>
am: add missing checks.

Signed-off-by: Daniel Stodden <daniel.stodden@gmail.com>