This should create a u-boot directory with all the latest sources.
Note that for more stability you might want to checkout a release instead of the latest version; to do so, list the available release tags with e.g. git tag -l 'v2*', and git checkout <the-desired-tag>.
Assuming your cross compiler is called e.g. arm-linux-gnueabihf-gcc, you can compile by doing:
$ cd u-boot
$ export CROSS_COMPILE=arm-linux-gnueabihf-
$ make mx6qsabresd_config
This should create a number of files, including u-boot.imx.
Put on SD
U-boot should reside at offset 1024B of your SD card. To put it there, do:
Your SD card device is typically something in /dev/sd<X> or /dev/mmcblk<X>. Note that you need write permissions on the SD card for the command to succeed, so you might need to su - as root, or use sudo, or do a chmod a+w as root on the SD card device node to grant permissions to users.
Your SD card is ready for booting. Insert it in the SD card slot of your i.MX6 sabre sd platform, connect to the USB to UART port with a serial terminal set to 115200 baud, no parity, 8bit data, power up the platform and you should see something like: