Makefile.lib: find capsule ESL dtsi file with CONFIG_OF_UPSTREAM

When CONFIG_OF_UPSTREAM is enabled, DTS files are in SOC subdirectories (vs the
top level dts directory), but when CONFIG_EFI_CAPSULE_AUTHENTICATE is enabled,
the dynamically created dtsi file containing the capsule ESL DT node is in the
parent directory. This results in a build failure because the #include inserted
in the DTS file is local to the current directory.  Update Makefile to have the
DT preprocessing of #includes search in the parent (dts top level) directory
too.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>

scripts/Makefile.lib

@@ -334,7 +334,7 @@ cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 	(cat $< > $(pre-tmp)); \
 	$(foreach f,$(subst $(quote),,$(dtsi_include_list)), \
 	  echo '$(pound)include "$(f)"' >> $(pre-tmp);) \
-	$(HOSTCC) -E $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $(pre-tmp) ; \
+	$(HOSTCC) -E $(dtc_cpp_flags) -I$(obj) -x assembler-with-cpp -o $(dtc-tmp) $(pre-tmp) ; \
 	$(DTC) -O dtb -o $@ -b 0 \
 		-i $(dir $<) -i $(u_boot_dtsi_loc) $(DTC_FLAGS) \
 		-d $(depfile).dtc.tmp $(dtc-tmp) || \