The block that controls the `.upload` target is unnecessarily replicated in multiple sub-board Makefiles. This was originally done because the SmartRF and the Launchpad can be programmed with the c2538-bsl script, whereas the sensortag cannot.
This commit moves the `cc2538-bsl` / `.upload` target logic to the top level cpu Makefile (`cpu/cc26xx-cc13xx/Makefile.cc26xx-cc13xx`). Board makefiles simply set the make variable `BOARD_SUPPORTS_BSL` to 1 to signal that they can be programmed by the BSL script. If `BOARD_SUPPORTS_BSL` is not equal to 1, trying to use the `.upload` target will return an error message.
For example:
```
$ make BOARD=sensortag/cc2650 cc26xx-demo.upload
using saved target 'srf06-cc26xx'
This board cannot be programmed with the ROM bootloader and therefore does not support the .upload target.
```
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