imxrt/mkimg: Provide valid example
# $2 - kernel argument string
# $3 - output file name
# $4, ... - applications ELF(s)
-# example: ./mkimg-imxrt.sh phoenix-armv7-imxrt.elf "Xapp1.elf,Xapp2.elf" flash.img app1.elf app2.elf
+# example: ./mkimg-imxrt.sh phoenix-armv7-imxrt.elf "Xapp1.elf Xapp2.elf" flash.img app1.elf app2.elf
reverse() {
