uboot-at91: fix build on buildbots

uboot-at91: fix build on buildbots

Buidbots are throwing the following compile error:

In file included from tools/aisimage.c:9:
include/image.h:1133:12: fatal error: openssl/evp.h: No such file or directory
            ^~~~~~~~~~~~~~~
compilation terminated.

Fix it by passing `UBOOT_MAKE_FLAGS` variable to make.

Suggested-by: Petr Štetiar <ynezz@true.cz>
Fixes: 6d5611af2813 ("uboot-at91: update to linux4sam-2022.04")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>