When waiting for transmission (to CC2420) to be done,
we were wrongly waiting for UCBUSY bit to be set,
while one should actually wait for that bit to be cleared.
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").
We have sane defaults for `ELFFILE` and `HEXFILE` in the root
`Makefile.include`. The local definition for `ELFFILE` of mbed_lpc1768's
`Makefile.include` was wrong, which caused e.g. `make buildsize` to
fail.
