udev: update init script for conditional probing of platform bus

udev: update init script for conditional probing of platform bus

Make probing of "platform" bus conditional based on PROBE_PLATFORM_BUS
variable from /etc/default/udev-cache on subsequent boots when udev
cache is used. PROBE_PLATFORM_BUS has to be set to "yes" in order to
force probing on every boot, otherwise it uses the old default behaviour
of probing it just the first time.

This is helpful on modern SoCs where most of the low level peripheral
drivers are registered on the platform bus and need to be probed to load
the necessary modules and enable the connected buses and subsystems.

(From OE-Core rev: 70a695735e0a7d14448f2f5a9986bfe105210a91)

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>