boot/uboot: fix environment image generation on big endian systems

boot/uboot: fix environment image generation on big endian systems

As reported by Jeff Wittrock in bug #11396, the U-Boot environment
image checksum is invalid for big endian targets, because the test on
the BR2_ENDIAN Config.in option doesn't take into account that it is
double quoted.

The fix was provided by Jeff himself on bugzilla.

Fixes bug #11396.

Reported-by: Jeff Wittrock <jwittrock@faultrecorder.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>