u-boot: Fix condition for install_spl_helper

u-boot: Fix condition for install_spl_helper

The condition for calling install_spl_helper when compiling multiple
configs does not match the condition for a single config. This causes
compilation failures when ${UBOOT_FITIMAGE_ENABLE} is 1 but
${SPL_SIGN_ENABLE} is not.

Fixes: 5af4dfe83c2 ("u-boot: Add infrastructure to SPL verified boot")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>