Implement set_string in CLI client

We can now set strings in a given (named) layer, and observe they are
indeed stored to disk. The following commit will implement get_string,
before we change the returns of the lbuxton utility methods to use
enums, so that we can identify errors (note that an unintentional cast
from bool to int is happening, affecting return code from buxtonctl)

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>