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_0_w_defconfig b/configs/rpi_0_w_defconfig
index 5da334a..7554e67 100644
--- a/configs/rpi_0_w_defconfig
+++ b/configs/rpi_0_w_defconfig
@@ -12,6 +12,7 @@
 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