sunxi: Normalise FEL support

Make sunxi's FEL code fit with the normal U-Boot boot sequence instead of
creating its own. There are some #ifdefs required in start.S. Future work
will hopefully remove these.

This series is available at u-boot-dm, branch sunxi-working.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
diff --git a/board/sunxi/Kconfig b/board/sunxi/Kconfig
index 4a21589..3eab81f 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -149,6 +149,16 @@
 	bool "SPL/FEL mode support"
 	depends on SPL
 	default n
+	help
+	  This enables support for Fast Early Loader (FEL) mode. This
+	  allows U-Boot to be loaded to the board over USB by the on-chip
+	  boot rom. U-Boot should be sent in two parts: SPL first, with
+	  'fel write 0x2000 u-boot-spl.bin; fel exe 0x2000' then U-Boot with
+	  'fel write 0x4a000000 u-boot.bin; fel exe 0x4a000000'. This option
+	  shrinks the amount of SRAM available to SPL, so only enable it if
+	  you need FEL. Note that enabling this option only allows FEL to be
+	  used; it is still possible to boot U-Boot from boot media. U-Boot
+	  SPL detects when it is being loaded using FEL.
 
 config UART0_PORT_F
 	bool "UART0 on MicroSD breakout board"