common: convert compulab splash load code to common code

Move board/compulab/common/splash.c code to
common/splash_source.c to make it available for everybody. This move
renames cl_splash_screen_prepare() to splash_source_load(), and
the compilation of this code is conditional on CONFIG_SPLASH_SOURCE.

splash_source features:
* Provide a standardized way for declaring board specific splash screen
  locations
* Provide existing routines for auto loading the splash image from the
  locations as declared by the board
* Introduce the "splashsource" environment variable, which makes it
  possible to select the splash image source.

cm-t35 and cm-fx6 are updated to use the modified version.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/board/compulab/cm_t35/cm_t35.c b/board/compulab/cm_t35/cm_t35.c
index 8143c05..c4ea8ea 100644
--- a/board/compulab/cm_t35/cm_t35.c
+++ b/board/compulab/cm_t35/cm_t35.c
@@ -19,6 +19,7 @@
 #include <i2c.h>
 #include <usb.h>
 #include <mmc.h>
+#include <splash.h>
 #include <twl4030.h>
 #include <linux/compiler.h>
 
@@ -69,8 +70,8 @@
 
 int splash_screen_prepare(void)
 {
-	return cl_splash_screen_prepare(splash_locations,
-					ARRAY_SIZE(splash_locations));
+	return splash_source_load(splash_locations,
+				  ARRAY_SIZE(splash_locations));
 }
 
 /*