package/spidermonkey: do not build the JavaScript shell, by default

Add a configuration to enable the JavaScript shell (default off). So
far only libmozjs is required (by polkit) and the shell takes around
24MiB.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>