Bluetooth: Mesh: shell: Fix parsing "boolean" parameters

The shell was being inconsistent in its parsing of boolean parameters.
Some commands were documented as accepting "on" but were actually
parsing the parameter with strtoul() which would result in 0.
Introduce a new helper to convert a string to a u8_t which still
accepts "on" or "enable". This gives us full flexibility of having a
simple interface to the user, but still allowing non-boolean values to
be tested (since on-air the value is a full octet).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>