spl: move RAM boot support in separate file

Add a new top-level config option so support booting an image stored
in RAM. This allows to move the RAM boot support into a sparate file
and having a single condition to compile that file.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/common/spl/Makefile b/common/spl/Makefile
index ed02635..1933cbd 100644
--- a/common/spl/Makefile
+++ b/common/spl/Makefile
@@ -26,4 +26,5 @@
 obj-$(CONFIG_SPL_SATA_SUPPORT) += spl_sata.o
 obj-$(CONFIG_SPL_DFU_SUPPORT) += spl_dfu.o
 obj-$(CONFIG_SPL_SPI_LOAD) += spl_spi.o
+obj-$(CONFIG_SPL_RAM_SUPPORT) += spl_ram.o
 endif