luci-base: override the load() function to return boolean for FlagValue

luci-base: override the load() function to return boolean for FlagValue

In the underlying uci system, all variables are effectively strings, so
for those configuration values which serve as 'boolean' flags, we need
to coerce the various forms into a real boolean. Only the following
string values result in true: 1|on|true|yes|enabled. Otherwise, false.

Checkboxes now fill correctly.

"Unchanged" configurations may write changed values as the Flag values
are coerced to '1' or '0' on write, but the configuration behaviour
remains synonymous.

Signed-off-by: Paul Donald <[email protected]>
(cherry picked from commit 1355a6fa253d765140f5571354939d2e93e6d967)