u-boot: Avoid building U-Boot-only binman images

For Rockchip boards U-Boot tries to build SPI and MMC images that
require an externally built BL31 file to be provided, and the build
fails otherwise. This is not really as configurable as it should be.

In Libreboot, we only care about the build outputs for U-Boot proper.
There is a BL31 built during our coreboot builds, but using that in
U-Boot builds is a chicken-and-egg problem. Building BL31 outside the
coreboot build and passing it to both projects is possible, but needs
work.

For now, stop trying to build these U-Boot-only images as a workaround,
by removing the binman image descriptions from the device-tree sources.
Additionally, disable in our configs the BINMAN_FDT functionality that
allows using these at runtime as it requires them to be present.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

config/u-boot/gru_bob/config/default

@@ -2175,7 +2175,7 @@ CONFIG_FS_FAT_MAX_CLUSTSIZE=65536
 # CONFIG_SYS_TIMER_COUNTS_DOWN is not set
 # CONFIG_PHYSMEM is not set
 # CONFIG_BCH is not set
-CONFIG_BINMAN_FDT=y
+# CONFIG_BINMAN_FDT is not set
 # CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED is not set
 CONFIG_CHARSET=y
 # CONFIG_DYNAMIC_CRC_TABLE is not set