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_4_defconfig b/configs/rpi_4_defconfig
index bbc0fd6..fa2c05d 100644
--- a/configs/rpi_4_defconfig
+++ b/configs/rpi_4_defconfig
@@ -10,6 +10,7 @@
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_DM_RESET=y
 CONFIG_SYS_LOAD_ADDR=0x1000000
+CONFIG_BOOTSTD_DEFAULTS=y
 CONFIG_PCI=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_DISTRO_DEFAULTS=y