Allow custom specification of the toolchain path

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Fix Makefile.config include path in example's Makefile

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Allow specification of the path to JlinkExe

This needs updated version of nrftool which add --jlinkexe option.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Allow specification of the programmer command

This change should allow easy change to for example 'nrftool --verbose' in my
custom Makefile.config.

Signed-off-by: Petr Štetiar <ynezz@true.cz>