Convert CONFIG_SYS_SPL_MALLOC_SIZE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPL_MALLOC_SIZE
   CONFIG_SYS_SPL_MALLOC_START

We introduce a default value here as well, and CONFIG_SYS_SPL_MALLOC to
control if we have a malloc pool or not.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/riotboard_defconfig b/configs/riotboard_defconfig
index 8e74a1f..8fe264d 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_RAW_IMAGE_SUPPORT=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot-dtb.img"
 CONFIG_SPL_FS_LOAD_ARGS_NAME="imx6dl-riotboard.dtb"