CRC generation: code cleaning.

Added an INITIAL_REMAINDER macro, changed the name of an increment variable.
CRC table generator: the hex numbers are now in upper case.
CRC unit tests: added two test cases.

Test case 1 compares the generated table with a table coming from
open source software, using the CRC-CITT standard.

Test case 2 compares the outputs given by our CRC algorithm and
the open source one.

Test case 3 verifies that, for a perfect transmission (no difference
between the sent and received messages), the CRC is zero.
Unit test Makefile: added an independent build directory.

This avoids mixing code compiled for the host and code compiled
for the target architecture.