Kconfig: Migrate all of cmd/fastboot/Kconfig to defconfigs

- Move ANDROID_IMAGE_SUPPORT to top level Kconfig under images as it's
  not strictly part of fastboot.
- Add some defaults for the fastboot buffer location and size
- Migrate all options listed in cmd/fastboot/Kconfig
- Cleanup the README

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/Kconfig b/Kconfig
index c48f00e..238fa3e 100644
--- a/Kconfig
+++ b/Kconfig
@@ -162,6 +162,13 @@
 
 menu "Boot images"
 
+config ANDROID_BOOT_IMAGE
+	bool "Enable support for Android Boot Images"
+	default y if FASTBOOT
+	help
+	  This enables support for booting images which use the Android
+	  image format header.
+
 config FIT
 	bool "Support Flattened Image Tree"
 	select MD5