common: cli_hush: Restore clear local variable support

The u-boot hush shell doesn’t support the unset command to clear a
variable and therefore an empty value ("c=") should be a valid value
for the set_local_var function to clear the variable. This partial
reverts commit aa722529 ("common: cli_hush: avoid dead code") and
only checks for a `=` in the string. Additionally explicit call the
unset_local_var function to remove the variable if the value is empty.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>