rockchip64: rename hook to board_uboot_spi_image_after_build

Per rpardini's review: the old name board_uboot_spi_image_postprocess
implied the hook postprocesses the SPI image itself. It does not —
rkspi_loader.img is already built when the hook fires; the hook only
lets the board ship extra files (e.g. spl_loader_maskrom.bin) in the
u-boot deb.

Rename to board_uboot_spi_image_after_build to reflect timing, matching
the board_uboot_spl_blobs_postprocess naming rpardini approved.

The Seeed extension implementation is renamed in the same commit on
Seeed-Studio/seeed_armbian_extension (b4fb0ce8).

Author: mingzhangqun

config/sources/families/include/rockchip64_common.inc

@@ -324,8 +324,8 @@ 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_postprocess >/dev/null && \
-			board_uboot_spi_image_postprocess
+		declare -F board_uboot_spi_image_after_build >/dev/null && \
+			board_uboot_spi_image_after_build
 	fi
 }