fastboot: Enable flashing by default on sunxi

Now that more and more devices are built using eMMC, providing a way to
easily flash the system without too much hassle seems like a right thing to
do.

Since fastboot is the most deployed tool to do that these days, we can just
rely on it to provide a way to flash the various components in the system
(SPL, U-Boot and the system itself) easily, especially since you can upload
the U-Boot hosting the fastboot "server" through FEL.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/cmd/fastboot/Kconfig b/cmd/fastboot/Kconfig
index 4ce7a77..cbb9183 100644
--- a/cmd/fastboot/Kconfig
+++ b/cmd/fastboot/Kconfig
@@ -66,6 +66,7 @@
 
 config FASTBOOT_FLASH
 	bool "Enable FASTBOOT FLASH command"
+	default y if ARCH_SUNXI
 	help
 	  The fastboot protocol includes a "flash" command for writing
 	  the downloaded image to a non-volatile storage device. Define