arm: rpi: Switch to standard boot

Drop use of the distro scripts and use standard boot instead.

We don't need to specify the mmc devices individually, since they are
used in order from 0 to 2, and standard boot uses that order anyway.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_3_defconfig b/configs/rpi_3_defconfig
index bb6fe12..a0d8be4 100644
--- a/configs/rpi_3_defconfig
+++ b/configs/rpi_3_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_SYS_LOAD_ADDR=0x1000000
+CONFIG_BOOTSTD_DEFAULTS=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_USE_PREBOOT=y