Consistent use of msp430-elf-objcopy in linker

[pmaene]

Found a remaining reference to msp430-objcopy. My guess is that there's no functional difference between msp430-objcopy and msp430-elf-objcopy for the given arguments, though.

[jovanbulck]

Thanks! Until we do a full migration to TI MSPGCC (issue #1), I'll say we stick with the usual MSPGCC toolchain whenever possible, however. Advantage is that the TI MSPGCC dependency is now only truly needed when using the optional --inline-arithmetic linker argument.

There is indeed no functional difference for this use. msp430-elf-objcpy is simply a more recent GNU binutils version, with support for the --add-symbol option. The latter is only needed for secure intra-SM arithmetics anyway.