Fix DTB conditional build regression

Only build DTB when ENABLE_SYSTEM=1 and ENABLE_ELF_LOADER=0, resolving
DTC syntax errors that occur when building with ELF loader mode enabled,
as CFLAGS_dt macros are only defined in kernel emulation mode.

The DTB is only needed for Linux kernel emulation, not for ELF loader
mode which loads bare-metal programs directly.

Author: jserv

Makefile

@@ -311,7 +311,14 @@ include mk/artifact.mk
 include mk/system.mk
 include mk/wasm.mk
 
-all: config $(BUILD_DTB) $(BUILD_DTB2C) $(BIN)
+DTB_DEPS :=
+ifeq ($(call has, SYSTEM), 1)
+ifeq ($(call has, ELF_LOADER), 0)
+DTB_DEPS := $(BUILD_DTB) $(BUILD_DTB2C)
+endif
+endif
+
+all: config $(DTB_DEPS) $(BIN)
 
 OBJS := \
     map.o \