[Buildroot] [PATCH] boot/uboot: fix uboot build failure with UBOOT_CUSTOM_DTS_PATH on uboot version >= 2020.x

Starting from version 2020.x uboot can't build .dts files not listed in
dts/Makefile leading to a build failure when trying to pass a .dts file to
UBOOT_CUSTOM_DTS_PATH. So let's prepend that file(s) to dts/Makefile if
UBOOT_CUSTOM_DTS_PATH is used.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
---
  boot/uboot/uboot.mk | 5 +++++
  1 file changed, 5 insertions(+)

diff --git a/boot/uboot/uboot.mk b/boot/uboot/uboot.mk
index 2478a2a1e9..a5ad101d68 100644
--- a/boot/uboot/uboot.mk
+++ b/boot/uboot/uboot.mk

@@ -303,6 +303,11 @@ endif # BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY
  UBOOT_CUSTOM_DTS_PATH = $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH))
  
  define UBOOT_BUILD_CMDS
+	$(if $(UBOOT_CUSTOM_DTS_PATH),
+		$(Q)$(SED) \
+		'1s;^;dtb-y += $(subst .dts,.dtb,$(call notdir,$(UBOOT_CUSTOM_DTS_PATH)))\n;' \
+		$(@D)/arch/$(UBOOT_ARCH)/dts/Makefile
+	)
  	$(if $(UBOOT_CUSTOM_DTS_PATH),
  		cp -f $(UBOOT_CUSTOM_DTS_PATH) $(@D)/arch/$(UBOOT_ARCH)/dts/
  	)