u-boot: Fix u-boot signing when building with multiple u-boot configs

The prepended code with the i and j variables clash with similar code in uboot.inc, so they need to be unset once we are done using them.

This commit fixes the issue that was introduced in d6858c9 "u-boot: Rework signing to remove interdependencies".

Signed-off-by: David Bagonyi <david.bagonyi@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>