uboot-imx: pico-pi-imx7d: fix wrong make flags overriding

uboot-imx: pico-pi-imx7d: fix wrong make flags overriding

Buidbots are currently choking on 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
  #  include <openssl/evp.h>
             ^~~~~~~~~~~~~~~
 compilation terminated.

This is caused by a complete overriding of make flags which are provided
correctly in `UBOOT_MAKE_FLAGS` variable, but currently overriden
instead of extended. This then leads to the usage of build host include
dirs, which are not available.

Fix it by extending `UBOOT_MAKE_FLAGS` variable like it was done in
commit 481339a04266 ("uboot-imx: fix wrong make flags overriding").

Fixes: 7094e6550336 ("uboot-imx: add support for TechNexion PICO-PI-IMX7D")
Signed-off-by: Petr Štetiar <ynezz@true.cz>