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/doc/README.splashprepare b/doc/README.splashprepare
index 61b4ec5..a0f0f3c 100644
--- a/doc/README.splashprepare
+++ b/doc/README.splashprepare
@@ -6,3 +6,16 @@
 sequence. It gives the board an opportunity to prepare the splash
 image data before it is processed and sent to the frame buffer by
 U-Boot.  Define your own version to use this feature.
+
+CONFIG_SPLASH_SOURCE
+
+Use the splash_source.c library. This library provides facilities to declare
+board specific splash image locations, routines for loading splash image from
+supported locations, and a way of controlling the selected splash location
+using the "splashsource" environment variable.
+
+splashsource works as follows:
+- If splashsource is set to a supported location name as defined by board code,
+  use that splash location.
+- If splashsource is undefined, use the first splash location as default.
+- If splashsource is set to an unsupported value, do not load a splash screen.