common: Makefile: Add rule to compile splash and splash_source at SPL

To enable splash screen and loading bmp from boot media, add rules to
compile splash.c and splash_source.c at SPL stage only when
CONFIG_SPL_SPLASH_SCREEN and CONFIG_SPL_SPLASH_SOURCE are defined.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>
Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>