gdb_server: use more local variables in inner loop of fetching packetstiny refactorin...

gdb_server: use more local variables in inner loop of fetching packetstiny refactoring to allow optimisation of inner loops

Some profiling information for arm7 16MHz GDB load operation shows
gdb_get_packet_inner() near the very top.

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  Ts/call  Ts/call  name
 52.91      2.27     2.27                             embeddedice_write_dcc
 11.89      2.78     0.51                             gdb_get_packet_inner
  8.86      3.16     0.38                             memcpy
  3.26      3.30     0.14                             idle_thread_main(unsigned int)
  3.03      3.43     0.13                             cyg_in_cksum

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>