learning uboot distro design in am335x-evm board

reference: 

　　uboot_dir/doc/README.distro

Linux distributions are faced with supporting a variety of boot mechanisms,
environments or bootloaders (PC BIOS, EFI, U-Boot, Barebox, ...). This makes
life complicated. Worse, bootloaders such as U-Boot have a configurable set
of features, and each board chooses to enable a different set of features.
Hence, distros typically need to have board-specific knowledge in order to
set up a bootable system.

　　That's why it happens.

in am335x uboot-2018.01, the follow code,very detailed describe the boot priority;

　　

#define CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG

#define BOOTENV_DEV_LEGACY_MMC(devtypeu, devtypel, instance)
"bootcmd_" #devtypel #instance "="
"setenv mmcdev " #instance"; "
"setenv bootpart " #instance":2 ; "
"run mmcboot"

#define BOOTENV_DEV_NAME_LEGACY_MMC(devtypeu, devtypel, instance)
#devtypel #instance " "

#define BOOTENV_DEV_NAND(devtypeu, devtypel, instance)
"bootcmd_" #devtypel "="
"run nandboot"

#define BOOTENV_DEV_NAME_NAND(devtypeu, devtypel, instance)
#devtypel #instance " "

#define BOOT_TARGET_DEVICES(func)
func(MMC, mmc, 0)
func(LEGACY_MMC, legacy_mmc, 0)
func(MMC, mmc, 1)
func(LEGACY_MMC, legacy_mmc, 1)
func(NAND, nand, 0)
func(PXE, pxe, na)
func(DHCP, dhcp, na)

#include <config_distro_bootcmd.h>

Note: when pcb board support boot from emmc and sdcard.

         cpu hardware design default emmc bootup was priority  than sdcard bootup.

         we can design sdcard was priority bootup than emmc under distro framwork ;

　　  so we can make sdcard bootup to upgrade emmc system in normal bootup.

　　  through sdcard burn image to emmc stroage.