drivers/cc2420: fix logical error in cc2420_do_send function

Do the correct test in cc2420_do_send() function when analyzing
the result of cc2420_load_tx_buf()
boards/xx/driver_cc2420: fix race condition in CCA determination

Ensure that CS is active and IRQ disabled when querying CCA
on CC2420 (cc2420_get_cca() function).
Also do a little refactoring (renamed c variable into count,
and named the constant corresponding to RSSI "timeout").
Merge pull request #1383 from rousselk/fix-cc2420-tx

Fix a design error in cc2420_do_send() function
Merge pull request #1385 from rousselk/fix-cc2420-driver-cca

Fix a nasty race condition in CCA determination on CC2420
Merge remote-tracking branch 'upstream/master' into cbor