Fix DTC build regression

Root cause: $(BUILD_DTB) had $(DTC_SENTINEL) as a prerequisite, causing
the recipe's $^ to expand to both src/devices/minimal.dts and src/dtc/.git.
The preprocessor then received two input files instead of one, breaking
the device tree compilation with "Error: <stdin>:69.9-11 syntax error".

Solution: Remove $(DTC_SENTINEL) from $(BUILD_DTB) dependencies. The
BUILD_DTB target uses the system dtc command and doesn't require the
submodule. Only BUILD_DTB2C needs the DTC_SENTINEL dependency for
libfdt headers.

Author: jserv

Makefile

@@ -201,7 +201,8 @@ ifeq ($(CONFIG_SYSTEM),y)
 ifneq ($(CONFIG_ELF_LOADER),y)
 DTB_DEPS := $(BUILD_DTB) $(BUILD_DTB2C)
 # Ensure DTC is cloned before building DTB
-$(BUILD_DTB) $(BUILD_DTB2C): $(DTC_SENTINEL)
+# Note: BUILD_DTB uses system dtc, only BUILD_DTB2C needs submodule headers
+$(BUILD_DTB2C): $(DTC_SENTINEL)
 endif
 endif