env: Provide a work-around for unquoting fdtfile

Some boards use a CONFIG option to specify the value of this variable.
This is normally handled by efi_get_distro_fdt_name() but in the case
of sunxi this does not work, since 'soc' is sunxi, but the files are
in the allwinner directory.

Provide a work-around for this particular case.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1844,6 +1844,7 @@ quiet_cmd_gen_envp = ENVP    $@
 		$(CPP) -P $(cpp_flags) -x assembler-with-cpp -undef \
 			-D__ASSEMBLY__ \
 			-D__UBOOT_CONFIG__ \
+			-DDEFAULT_DEVICE_TREE=$(subst ",,$(CONFIG_DEFAULT_DEVICE_TREE)) \
 			-I . -I include -I $(srctree)/include \
 			-include linux/kconfig.h -include include/config.h \
 			-I$(srctree)/arch/$(ARCH)/include \

doc/usage/environment.rst

@@ -87,6 +87,18 @@ settings. For example::
 
    #include <env/ti/mmc.env>
 
+Quotes are not supressed, for example::
+
+    fdtfile=CONFIG_DEFAULT_DEVICE_TREE.dtb
+    # produces: fdtfile="sun7i-a20-pcduino3.dtb"
+
+For this particular issue you can use DEFAULT_DEVICE_TREE instead::
+
+    fdtfile=DEFAULT_DEVICE_TREE.dtb
+    # produces: fdtfile=sun7i-a20-pcduino3.dtb
+
+There is no general way to remove quotes.
+
 If CONFIG_ENV_SOURCE_FILE is empty and the default filename is not present, then
 the old-style C environment is used instead. See below.