stm32f429: pass the device unique ID in DTB

stm32f429: pass the device unique ID in DTB

Read device unique ID and set environment variable "serial#".
Value would then be passed to kernel through DTB.

To read ID from DTB, kernel is required to have commit:
3f599875e5202986b350618a617527ab441bf206 (ARM: 8355/1: arch: Show
the serial number from devicetree in cpuinfo)
This commit is already mainline since v4.1-rc1.

Signed-off-by: Antonio Borneo <[email protected]>
To: Albert Aribaud <[email protected]>
To: Tom Rini <[email protected]>
To: Kamil Lulko <[email protected]>
Cc: [email protected]