* Add pico_bootsel_via_double_reset library, fix#87
* pico_bootsel_via_double_reset: remove unnecessary library guard, fix subdirectory ordering
* Fix C type used as PICO_CONFIG type
In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."
Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.
