envsetup.sh: Add 'provision' function.

This will run ${ANDROID_PRODUCT_OUT}/provision-device which in turn can
run fastboot (and other vendor-specific bits, if needed) to repartition
the device and reflash all partitions with known versions. Options
passed to the 'provision' command will be passed on to said script which
is expected to pass them on to fastboot.

Since this is a rather destructive operation involving data loss etc.,
user confirmation is required. This confirmation can be bypassed using
the option --no-confirmation.

Change-Id: Iec5667f2f5070b95d2eb70ad1b2984909ae4184c