Relax UBOOT_ENTRYPOINT and UBOOT_LOADADDRESS default definitions

Set UBOOT_ENTRYPOINT and UBOOT_LOADADDRESS using the weak assignment operator (?=)
to allow for easier override in downstream layers or machine configurations
without forcing a hardcoded value.

Author: embetrix

recipes-kernel/linux/linux-raspberrypi.inc

@@ -44,5 +44,5 @@ KERNEL_MODULE_AUTOLOAD += "${@bb.utils.contains("MACHINE_FEATURES", "pitft28r",
 # manually. This value unused if KERNEL_IMAGETYPE is not uImage.
 KERNEL_EXTRA_ARGS += "LOADADDR=${UBOOT_ENTRYPOINT}"
 
-UBOOT_ENTRYPOINT =       "0x00008000"
-UBOOT_LOADADDRESS =      "0x00008000"
+UBOOT_ENTRYPOINT ?=       "0x00008000"
+UBOOT_LOADADDRESS ?=      "0x00008000"