cc2538: Generate .hex output

ELF files generated by GCC make SmartRF Flash Programmer 2 crash (only the TI
format is supported by this tool for ELFs), and binary files are not very
appropriate because they are gapless, so generate Intel HEX files since these
are very well supported by most programming tools while still flexible.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: Make it possible to use a custom linker script

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538dk: Make it possible to override UIP_CONF_TCP_MSS

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>