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.
