lib: provide clearer devicetree semantics

The devicetree data structures have been available in more than just
ramstage and romstage. In order to provide clearer and consistent
semantics two new macros are provided:

1. DEVTREE_EARLY which is true when !ENV_RAMSTAGE
2. DEVTREE_CONST as a replacment for ROMSTAGE_CONST

The ROMSTAGE_CONST attribute is used in the source code to mark
the devicetree data structures as const in early stages even though
it's not just romstage. Therefore, rename the attribute to
DEVTREE_CONST as that's the actual usage. The only place where the
usage was not devicetree related is console_loglevel, but the same
name was used for consistency. Any stage that is not ramstage has
the const C attribute applied when DEVTREE_CONST is used.

Change-Id: Ibd51c2628dc8f68e0896974f7e4e7c8588d333ed
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19333
Tested-by: build bot (Jenkins)
Reviewed-by: Philippe Mathieu-Daudé <philippe.mathieu.daude@gmail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/device/resource.h

@@ -42,7 +42,7 @@ struct resource {
 	resource_t base;	/* Base address of the resource */
 	resource_t size;	/* Size of the resource */
 	resource_t limit;	/* Largest valid value base + size -1 */
-	ROMSTAGE_CONST struct resource *next;	/* Next resource in the list */
+	DEVTREE_CONST struct resource *next;	/* Next resource in the list */
 	unsigned long flags;	/* Descriptions of the kind of resource */
 	unsigned long index;	/* Bus specific per device resource id */
 	unsigned char align;	/* Required alignment (log 2) of the resource */