cmd/setexpr: support concatenation of direct strings

cmd/setexpr: support concatenation of direct strings

The setexpr.s command allows to concatenate two strings.

According to the description in doc/usage/cmd/setexpr.rst the parameters
value1 and value2 can be either direct values or pointers to a
memory location holding the values.

Unfortunately `setexpr.s <value1> + <value2>` fails if any of the values
is a direct value. $? is set to false.

* Add support for direct values in setexpr.s.
* Correct the unit test for "setexpr.s fred 0".
* Add a new unit test for "setexpr.s fred '1' + '3'" giving '13'.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>