nx: Use prebuilt U-Boot for now

* CTC's 2018 u-boot is more feature-complete for nx for now.

* We may revisit this later.

* This remediates issues with RAM carveout, clocks
  (on erista models), mod chip issues (on mariko models),
  and brings us almost in-line with L4t releases for nx.

Change-Id: I622cc67acb543b6ac54c8ee7a585d88e8066606c

BoardConfig.mk

@@ -78,8 +78,10 @@ include device/nintendo/nx/modules.mk
 
 # Kernel Image Parameters
 BOARD_KERNEL_IMAGE_NAME        := Image.gz
-BOARD_KERNEL_LOAD_BASE         := 0x88000000
-BOARD_MKBOOTIMG_ARGS           := --base $(BOARD_KERNEL_LOAD_BASE)
+BOARD_MKBOOTIMG_ARGS           := \
+    --base 0x80000000 \
+    --kernel_offset 0x200000 \
+    --ramdisk_offset 0x4000000
 
 # Recovery
 TARGET_RECOVERY_DENSITY      := hdpi