sama5d4ek: trivial: reorder UBOOT_MACHINE line

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
sama5d4ek: use the existing bootloaders.inc file for EXTRA_IMAGEDEPENDS

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
sama5d3xek: remove not needed EXTRA_IMAGEDEPENDS we use bootloaders.inc

The include/bootloaders.inc file was created just for containing this
EXTRA_IMAGEDEPENDS directive for all AT91 machines: use it.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
at91bootstrap: update to 3.7 for all supported machines

As the 3.7 revision supports all listed machines, we update the commit ID for
all of them.
This solves an issue with do_deploy task on non-sama5d4 machines.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>