Depending on the linker script, the generated .bin file may start beyond
the beginning of the flash memory. However, no target address was passed
to cc2538-bsl.py by the upload make target, so it used the beginning of
the flash memory in all cases.
The load address of the lowest loadable output section is now passed to
cc2538-bsl.py. The start address of the .text output section or the
address of the _text symbol could have been used too, but this would not
have been compatible with all the possible custom linker scripts.
Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>