Update to match current binding expectations

Update to match current binding expectations

Changes:
- Reworked header, footer, message structure definition, etc. to
  match the contents of the current generated files in the binding.
- To enable the above, added some code to derive the API name for
  the app controller CAPI macro from the name given for the message
  set in the JSON. The name is lower-cased and spaces converted to
  dashes, this matches all the examples in the generated files in
  the binding.
- Added logic to default message length to 8 if not set in JSON,
  this is required to match explicit checks and signal JSON changes
  in the binding.
- Replaced is_bigendian and is_signed fields with new sign and
  bit_sign_position fields in JSON parsing and generated signal
  output. The sign field value enum has been copied from the binding
  to provide a reference for the values.
- Update per-signal flags generation, to enable this updated flag
  #defines from binding's current can-message.hpp. Added a comment
  to document which flags are known to be used.
- Removed "package" from "all" target in Makefile, as it is attached
  to widget packaging rules that are not applicable for the
  generator, which is built for host or toolchain usage.
- Removed unused src/main.hpp header to avoid confusion.

Bug-AGL: SPEC-3551

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I9ef3df0ebcfbddc23c789b50e1575fc42031970d
(cherry picked from commit 0a3e354c3d81866e1a755367ab5592b3ced868bb)