compulab: splash: support multiple splash sources

Define a generic way for boards to define splash image locations:
- introduce struct splash_location
- introduce enum splash_storage
- update cl_splash_screen_prepare() to take an array of above struct
  and select the appropriate one based on the splashsource environment
  variable (if it is not defined- use the first splash location as default).

cm-t35 is updated to work with the new interface.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>