If you want to boot LE with microSD on Banana Pi M5/M2Pro,
please read the following BPI wiki before do it.
Erase EMMC for SDcard Bootup
If Android is remained in eMMC, LE with microSD is not able to boot.
Like I was
If Android is installed on eMMC you need to perform recovery boot with the AMLGX "box" image so s905_autoscript or boot.scr can be found in the boot partition. The vendor u-boot looks for these files and loads them; and they reconfigure u-boot to find/boot the LE kernel and device-tree files first (if present) else continue to Android. Once you've booted into LE this way you can either erase/zero emmc using emmctool so the device runs u-boot from SD, or you can download the AMLGX "bananapi-m5.img.gz" image file to /storage and write the image to emmc (again using emmctool). Then the device is booting upstream u-boot from eMMC, but this will look for the .conf on SD before eMMC to make testing from SD easier, else run the image from eMMC which is faster.
