SPL: Move the omap SPL framework to common/spl

Add a new flag, CONFIG_SPL_FRAMEWORK to opt into the common/spl SPL
framework, enable on all of the previously using boards.  We move the
spl_ymodem.c portion to common/ and spl_mmc.c to drivers/mmc/.  We leave
the NAND one in-place as we plan to replace it later in this series.

We use common/spl to avoid linker problems with respect to merging
constant strings in objects.   Otherwise all strings in common/ will be
linked in and kept which grows SPL in size too much.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index 309d88d..c5e6a1a 100644
--- a/README
+++ b/README
@@ -2597,6 +2597,11 @@
 		CONFIG_SYS_SPL_MALLOC_SIZE
 		The size of the malloc pool used in SPL.
 
+		CONFIG_SPL_FRAMEWORK
+		Enable the SPL framework under common/.  This framework
+		supports MMC, NAND and YMODEM loading of U-Boot and NAND
+		NAND loading of the Linux Kernel.
+
 		CONFIG_SPL_DISPLAY_PRINT
 		For ARM, enable an optional function to print more information
 		about the running system.