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/mx6cuboxi_defconfig b/configs/mx6cuboxi_defconfig
index 41122ad..d68335b 100644
--- a/configs/mx6cuboxi_defconfig
+++ b/configs/mx6cuboxi_defconfig
@@ -26,6 +26,7 @@
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="if hdmidet; then usb start; setenv stdin  serial,usbkbd; setenv stdout serial,vidconsole; setenv stderr serial,vidconsole; else setenv stdin  serial; setenv stdout serial; setenv stderr serial; fi;"
 CONFIG_BOARD_EARLY_INIT_F=y
+CONFIG_SYS_SPL_MALLOC=y
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_FS_LOAD_PAYLOAD_NAME="u-boot-dtb.img"
 CONFIG_SPL_I2C=y