This commit allows to enable/disable the txtsnd shell command. The
command is used to send strings over L2 in GNRC. Until now the command
was automatically enabled if GNRC and shell_commands were present, which
may lead to confusion if no L2 packet handler is registered.
squash: driver: fixed typo to pass doccheck
squash: driver: bug fixes after test
squash: adapted Makefiles for correct build behaviour
restructured, no spi interaction in isr anymore
major bugfixes and restructurization
comments addressed
introduce new netconf option, ..AUTOCCA
squash: minor bugfix and add auto-init mechanism
squash: minor fixes to make travis happy