kconfig: implement KCONFIG_PROBABILITY for rand{,package}config

rand{,package}config enables configuration options with 50% probability,
which isn't always what we want in BR (because the "big" configs take a
long time to build and limits the chance of a randconfig detecting a
missing dependency), so introduce a KCONFIG_PROBABILITY environment
variable to tweak the probability between 0 (all options off) and 100
(all options on).

To use, simply set the environment variable before running make or pass
it on the make command line - E.G.

make randpackageconfig KCONFIG_PROBABILITY=10

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>