rename symbol: CONFIG_KIRKWOOD -> CONFIG_ARCH_KIRKWOOD

Have this symbol follow the pattern of all other such symbols.
This patch also removes a TODO from the code.

Reviewed-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Trevor Woerner <twoerner@gmail.com>
diff --git a/Kconfig b/Kconfig
index 15f1a75..0e7ccc0 100644
--- a/Kconfig
+++ b/Kconfig
@@ -268,7 +268,7 @@
 	default "u-boot-elf.srec" if RCAR_GEN3
 	default "u-boot.itb" if SPL_LOAD_FIT && (ARCH_ROCKCHIP || \
 				ARCH_SUNXI || RISCV || ARCH_ZYNQMP)
-	default "u-boot.kwb" if KIRKWOOD
+	default "u-boot.kwb" if ARCH_KIRKWOOD
 	default "u-boot-with-spl.bin" if ARCH_AT91 && SPL_NAND_SUPPORT
 	default "u-boot-with-spl.imx" if ARCH_MX6 && SPL
 	help