Add U-Boot for stm32mp machine

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>

recipes-bsp/u-boot/u-boot-stm32mp-extlinux/boot.scr.cmd

@@ -0,0 +1,44 @@
+# Generate boot.scr.uimg:
+# ./tools/mkimage -C none -A arm -T script -d boot.src.cmd boot.scr.uimg
+#
+
+# M4 Firmware load
+env set m4fw_name "rproc-m4-fw.elf"
+env set m4fw_addr ${kernel_addr_r}
+env set boot_m4fw 'rproc init; rproc load 0 ${m4fw_addr} ${filesize}; rproc load_rsc 0 ${m4fw_addr} ${filesize}; rproc start 0'
+
+# boot M4 Firmware when available
+env set scan_m4fw 'if test -e ${devtype} ${devnum}:${distro_bootpart} ${m4fw_name};then echo Found M4 FW $m4fw_name; if load ${devtype} ${devnum}:${distro_bootpart} ${m4fw_addr} ${m4fw_name}; then run boot_m4fw; fi; fi;'
+
+# Update DISTRO command= search in sub-directory and load M4 firmware
+env set boot_prefixes "/${boot_device}${boot_instance}_${board_name}_"
+env set boot_extlinux "run scan_m4fw;${boot_extlinux}"
+
+if test ${boot_device} = mmc; then
+    if test ${distro_bootpart} > 4; then
+        env set boot_prefixes "/mmc${boot_instance}_${board_name}-optee_"
+    fi
+
+    #start the correct exlinux.conf
+    run scan_dev_for_boot_part
+
+elif test ${boot_device} = nand; then
+
+    #start the correct exlinux.conf without remount UBI
+    run scan_dev_for_boot
+
+elif test ${boot_device} = nor; then
+
+    #SDCARD boot
+    run bootcmd_mmc0
+
+    #NAND boot
+    env set boot_prefixes "/nand0_${board_name}_"
+    run bootcmd_ubifs0
+
+    #EMMC boot
+    env set boot_prefixes "/${boot_device}${boot_instance}-mmc1_${board_name}_"
+    run bootcmd_mmc1
+fi
+
+echo SCRIPT FAILED... ${boot_prefixes}extlinux/extlinux.conf not found !