ARM: ti: consolidate mmc environment variables

Introduce include/environment/ti/mmc.h that
consolidates environment variable definitions
for various TI boards that support MMC/SD.

This allows reuse of same environment variables
on non-ARMv7 TI platforms like OMAP-L138 for
example.

While at it, move DFU-related environment variable
includes to only non-SPL builds for AM335x and
AM437x since they are not really used for SPL
today.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/configs/k2g_evm.h b/include/configs/k2g_evm.h
index 9e5949e..bee1be7 100644
--- a/include/configs/k2g_evm.h
+++ b/include/configs/k2g_evm.h
@@ -10,6 +10,7 @@
 #ifndef __CONFIG_K2G_EVM_H
 #define __CONFIG_K2G_EVM_H
 
+#include <environment/ti/mmc.h>
 #include <environment/ti/spi.h>
 
 /* Platform type */