make: remove local definitions of (ELF|HEX)FILE

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.
Merge pull request #1303 from Kijewski/mbed_lpc1768-fix-result-paths

make: remove local definitions for ELFFILE and HEXFILE (was: "mbed_lpc1768: fix ELF and HEX paths")