[D18][MACHINE] Added D18 board support

This merge request introduces full support for the SECO i.MX8M Plus D18 platform in the meta-seco-imx Yocto layer. Key changes include:

Machine Definition

• Added a new machine configuration: seco-imx8mp-d18.conf
• Defined:
  • SECO_SOC, SOC_FAMILY, and MACHINEOVERRIDES
  • Includes common and base configurations
  • U-Boot machine, bootloader images, kernel and console settings

Yconfig Extension

• Extended Yconfig_machine to define MACH_MACHINE_seco-imx8mp-d18, enabling configuration selection during build.

Environment Handling

• Added a custom U-Boot environment patch:
  0002-ENVIRONMENT-SECOIMX8MPD18-Change-environment-for-Clea-OS.patch
  This patch configures U-Boot environment variables to be Clea-OS specific.
• Introduced a new boot.env file under recipes-bsp/bootscript, which sets:
  • kernel=Image
  • devicetree=seco-imx8mp-d18.dtb
  • Console and partition parameters