debug boot: execute dmesg during early boot (2/2)

see system/core/init for commit (1/2)

logfiles can be found at:
system/dmesg.txt and /system/boot.txt

to use this, add these flags to BoardConfig.mk in device tree

DEBUG_BOOT := true
TARGET_SYSTEM_MINOR := 20 (this needs to match the system partition number of the device)

note: with selinux enforcing device can't boot up with this activated!