Place common OpenOCD configs for the different cc13xx and cc26xx
families in boards/common/cc26xx_cc13xx/dist and automatically select
the correct one based on the (prefix of the) value of `$(CPU_MODEL)`, if
`OPENOCD_CONFIG` was not specified and no custom `openocd.cfg` is
located in the board's `dist` folder.
The `cc2650-launchpad` and `cc2650stk` have been adapted to use the
common config instead.
Only default to uniflash as default programmer if UNIFLASH_PATH is
set. If this isn't the case, `make flash` fails complaining about
missing `UNIFLASH_PATH` anyway.
