Nokia RX-51: Move code from defconfig back to C header file

In commit commit 37304aaf ("Convert CONFIG_USE_PREBOOT and
CONFIG_PREBOOT to Kconfig") was moved complicated multiline script code
from C header to oneliner in defconfig. After this change multiline to wide
oneliner it is hard to read this code and even harder to debug. Moreover
this script code should be at place where are other scripts, so move it
back to C header file.

Define new env variable preboot which stores this script and in option
CONFIG_PREBOOT calls this preboot variable.

Signed-off-by: Pali Rohár <pali@kernel.org>