bootscript:alt: Check for custom script and run it if found

The Flash-N-Go System bootscript should also check for an additional
custom bootscript and run it if one was found.

recipes-bsp/bootscript/bootscript-uboot-alt/boot-alt.scr.cfg

 if fatload mmc ${current_partition} ${loadaddr} boot-alt.cfg  && env import -t ${loadaddr} ${filesize}; then
 
+    if test -n ${custom_boot_script}; then
+        echo "Running custom bootscript ..."
+        fatload mmc ${current_partition} ${loadaddr} ${custom_boot_script}
+        source ${loadaddr}
+    fi
+
     if test -n ${cmdline}; then
         echo "Kernel cmdline: ${cmdline}"
         setenv bootargs ${cmdline}