This common code can be used byt posix based boars to provide
   the board_identity API.

   The form of the PX4  GUID is as follows:
   offset:0         1         2         -           17
      <ARCH MSD><ARCH LSD><MSD CPU UUID>...<LSD CPU UUID>

   Where <ARCH MSD><ARCH LSD> are a monotonic ordinal number assigned by
   PX4 to a chip architecture (PX4_SOC_ARCH_ID). The 2 bytes are used to
   create a globally unique ID when prepended to a padded CPU ID.

   In the case where the CPU's UUID is shorter than 16 bytes it will be
   padded with 0's starting at offset [2] until
   PX4_CPU_MFGUID_BYTE_LENGTH-PX4_CPU_UUID_BYTE_LENGTH -1
   I.E. For the STM32
   offset:0         1     2  3  4  5  6             -            17
     <ARCH MSD><ARCH LSD>[0][0][0][0]<MSD CPU UUID>...<LSD CPU UUID>

   I.E. For as CPU with a 16 byte UUID
   offset:0         1         2         -           17
     <ARCH MSD><ARCH LSD><MSD CPU UUID>...<LSD CPU UUID>

desired such that it's possible to use it in manual modes

due to limited flash space.

A buffer size of the gps drivers got reduced by 100 bytes.

This uses now the same sleep time logic as mavlink, depending on the
baudrate.

CPU usage on a Pixracer for different sleep times:

#num reads/sec    sleep time         CPU usage
17-18             2.8ms              0.233-0.31% (this PR)
12                5ms                0.155-0.3%
9-10              10ms               0.155-0.233%
6                 20ms               0.155-0.233% (previous)