3.1.2 Build U-Boot from Sources

Perform the following steps to build the u-boot.

Note: Make sure to install the mkenvimage tool on the Linux machine.

Export the cross compiler toolchain path, using the following command:
```
export CROSS_COMPILER=arm-linux-gnueabi-
```
Clone the Linux4sam GitHub U-Boot repository, enter in to the cloned directory using the following command:
```
$ git clone git://github.com/linux4sam/u-boot-at91.git
```
Move to the u-boot-at91 directory, using the following command:
```
$ cd u-boot-at91
```
Switch to a new branch u-boot-2018.07-at91, using the following commands:
```
$ git branch -r
$ git checkout origin/u-boot-2018.07-at91 -b u-boot-2018.07-at91
```
The configs/ directory lists many defconfig files. Use the sama5d2_xplained_mmc_defconfig file to configure the u-boot by using the following command:
```
$make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- sama5d2_xplained_mmc_defconfig
```

Open the u-boot-at91/include/configs/sama5d2_xplained.h file and modify the definitions for FAT_ENV_DEVICE_AND_PART and CONFIG_BOOTCOMMAND, using the following commands:

/*bootstrap + u-boot + env in sd card */
#undef FAT_ENV_DEVICE_AND_PART 
#undef CONFIG_BOOTCOMMAND 
#define FAT_ENV_DEVICE_AND_PART    "0"
#define CONFIG_BOOTCOMMAND        "fatload mmc 0:1 0x21000000 at91-sama5d2_xplained.dtb; " \
                                  "fatload mmc 0:1 0x22000000 zImage; " \
                                  "bootz 0x22000000 - 0x21000000"
#undef CONFIG_BOOTARGS
#define CONFIG_BOOTARGS           "console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rw rootwait"
#elif CONFIG_SPI_BOOT

Build the u-boot binary, using the following command:
```
$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi-
```
- The result of these operations is a fresh U-Boot binary called u-boot.bin corresponding to the binary ELF file u-boot.
  - u-boot.bin file will be created in u-boot-at91 directory.
  - u-boot is the ELF format binary file can be used to debug U-Boot through a JTag link.

Create a text file named u-boot-env.txt in any directory with the u-boot environment variables mentioned below:

bootargs=console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait
bootcmd=fatload mmc 0:1 0x21000000 at91-sama5d2_xplained.dtb; fatload mmc 0:1 0x22000000
zImage; bootz 0x22000000 - 0x21000000
bootdelay=1
ethact=gmac0
stderr=serial
stdin=serial
stdout=serial

Move to the direcrory and enter the following command to generate uboot.env file from the previously created u-boot-env.txtfile.
```
$ mkenvimage -s 0x2000 -o uboot.env u-boot-env.txt
```