[ESD-2009] Turn on auto formatting

[woodfell]

Enable the clang-format cmake module and run during generation of C source/headers. .clang-format imported from Starling.

Adjust the C header template slightly to achieve the following:

• documentation comments are attached to the struct for a message rather than the #define for the message ID
• documentation comments for struct members are placed preceding the member rather than trailing to prevent excessive line breaks and poor formatting

Previous documentation structure was:

/* Documentation for struct
 *
 * xxx
 */
#define SBP_MSG_xxx (optional)
#define bitfield macros (optional)

typedef struct {
  u8 member; /**< Documentation for member which sometimes
* wraps like this
* [m/s] */
} xxx;

has been changed to

#define SBP_MSG_xxx (optional)
#define bitfield macros (optional)

/* Documentation for struct
 *
 * xxx
 */
typedef struct {
  /** 
   * Documentation for member which wraps
   * nicely [m/s]
   */
  u8 member;
} xxx;

There are no functional changes in this PR. The only places which were edited by hand are:

• Makefile
• c/CMakeLists.txt
• generator/sbpg/targets/resources/sbp_messages_template.h
• c/include/cpp/state.h (shuffling some // NOLINTs)

Suggestion for reviewing:
Review changes to Makefile, c/CMakeLists.txt, c/.clang-format, and generator/sbpg/targets/resources/sbp_messages_template.h. Copy them in to a fresh checkout of master and run make c. Then diff the repo to this branch to confirm that all other changes are as a result of clang-format.

[reimerix]

I think the current build image might not include clang-format, here is a snippet from running make -j8 c using the swiftnav/libsbp-build:2021.04.02 image on my machine:

CMake Warning at cmake/common/ClangFormat.cmake:122 (message):
  Could not find appropriate clang-format, targets disabled
Call Stack (most recent call first):
  cmake/common/ClangFormat.cmake:218 (early_exit)
  CMakeLists.txt:13 (swift_setup_clang_format)

[silverjam]

@woodfell please update the README.md/HOWTO.md to point to the new docker image tag

[reimerix]

LGTM

[jbangelo]

What's the motivation to remove the empty structs? I can't think of a reason to have included them in the first place, but removing them is a breaking change.

[woodfell]

What's the motivation to remove the empty structs? I can't think of a reason to have included them in the first place, but removing them is a breaking change.

Oops, getting ahead of myself here. Empty structs are one of the problems in libsbp but I'll put them back in for the moment

[denniszollo]

this is objectively more pretty before than it is after. It used to be visible in 5 lines, now it takes 25,.