rockchip64: fix errexit build failure on SPI boards (undefined hook)

Commit 944f2f9 added a board_uboot_spi_image_after_build hook using the
idiom:

    declare -F board_uboot_spi_image_after_build >/dev/null && \
        board_uboot_spi_image_after_build

No board defines this hook, so 'declare -F' returns 1 and the && chain
evaluates to exit status 1. As the last statement in the
BOOT_SUPPORT_SPI branch it becomes the return value of
uboot_custom_postprocess, and under the runner's 'set -e -o pipefail'
that aborts the build:

    Error 1 occurred in main shell at lib/functions/compilation/uboot.sh:286

This broke every rockchip board with BOOT_SUPPORT_SPI=yes (e.g.
orangepi5-ultra and other rk35xx SPI-boot boards); boards without SPI
support were unaffected.

Guard the optional hook with a proper 'if' so a missing hook no longer
propagates a non-zero exit, matching the style of the sibling
board_uboot_spl_blobs_postprocess hook.

Signed-off-by: Igor Pecovnik <igor@armbian.com>

Author: igorpecovnik

config/sources/families/include/rockchip64_common.inc

@@ -324,8 +324,9 @@ uboot_custom_postprocess() {
 		# (e.g. for Maskrom recovery). Invoked after both rkspi_loader.img
 		# build paths so the hook contract is consistent regardless of
 		# BOOT_SPI_RKSPI_LOADER.
-		declare -F board_uboot_spi_image_after_build >/dev/null && \
+		if declare -F board_uboot_spi_image_after_build >/dev/null; then
 			board_uboot_spi_image_after_build
+		fi
 	fi
 }