Pi3 USB boot support

General Discussions
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Update: the function that handles the file writing seems to be here.

Here’s a test i’m doing in case it lights up ideas in someone’s head:

U-Boot CosmOS > env set toto tata
U-Boot CosmOS > env print toto
toto=tata
U-Boot CosmOS > saveenv
Saving Environment to FAT... OK
U-Boot CosmOS > boot
6694592 bytes read in 47 ms (135.8 MiB/s)
## Booting kernel from Legacy Image at 00080000 ...
   Image Name:   Linux kernel (CosmOS)
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    6694528 Bytes = 6.4 MiB
   Load Address: 00008000
   Entry Point:  00008000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 2eff3a00
   Booting using the fdt blob at 0x2eff3a00
   Loading Kernel Image
   Using Device Tree in place at 2eff3a00, end 2f002f6b

Starting kernel ...

root@pi:~ fw_printenv | grep toto
root@pi:~ cat /etc/fw_env.config 
/boot/u-boot/uboot.env         0x0000 0x4000
/boot/u-boot/uboot-redund.env  0x0000 0x4000
root@pi:~ cat /etc/fstab
proc			/proc				proc	defaults			0	0
/dev/root		/					ext4	defaults,noatime	0	1
/dev/sda1		/boot/u-boot		vfat	defaults			0	2
/dev/sda4		/data				ext4	defaults			0	2

rpi_4_32b_defconfig

CONFIG_ENV_SIZE=0x4000
CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
CONFIG_ENV_FAT_INTERFACE="usb"
CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
CONFIG_SYS_RELOC_GD_ENV_ADDR=y
CONFIG_SYS_REDUNDAND_ENVIRONMENT=y