mbed_lpc1768: fix ELF and HEX paths

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.
make: remove local definitions of HEXFILE

We set the default value for `HEXFILE` and `ELFFILE` in root
`Makefile.include`. There is no need to set these variables in the
board's `Makefile.include`.