0003.stm32mp157f：通过SD卡升级zImage和rootfs，以及buildroot编译rootfs、SDK、linux

--------------------------------------------------------------------------------一、buildroot编译：--------------------------------------------------------------------------------1、修改fw_env.conf文件med@med:~/work/project/rootfs/buildroot-external-st$med@med:~/work/project/rootfs/buildroot-external-st$med@med:~/work/project/rootfs/buildroot-external-st$ gedit board/stmicroelectronics/stm32mp1/overlay-demo/etc/fw_env.configmed@med:~/work/project/rootfs/buildroot-external-st$/dev/mmcblk0p4 -0x2000 0x2000/dev/mmcblk0p4 -0x2000 0x20002、med@med:~/work/project/rootfs/buildroot$med@med:~/work/project/rootfs/buildroot$ mv dl/ dl.bakmed@med:~/work/project/rootfs/buildroot$med@med:~/work/project/rootfs/b