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/arch/arm/Kconfig b/arch/arm/Kconfig
index 53d0831..65805f1 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -693,6 +693,8 @@
 	select USB_STORAGE if DISTRO_DEFAULTS
 	select USB_KEYBOARD if DISTRO_DEFAULTS
 	select USE_TINY_PRINTF
+	imply CMD_FASTBOOT
+	imply FASTBOOT
 	imply FAT_WRITE
 	imply PRE_CONSOLE_BUFFER
 	imply SPL_GPIO_SUPPORT
@@ -702,6 +704,7 @@
 	imply SPL_MMC_SUPPORT if MMC
 	imply SPL_POWER_SUPPORT
 	imply SPL_SERIAL_SUPPORT
+	imply USB_FUNCTION_FASTBOOT
 
 config TARGET_TS4600
 	bool "Support TS4600"
@@ -1080,7 +1083,10 @@
 	select DM_USB if USB
 	select DM_PWM
 	select DM_REGULATOR
+	imply CMD_FASTBOOT
+	imply FASTBOOT
 	imply FAT_WRITE
+	imply USB_FUNCTION_FASTBOOT
 
 config TARGET_THUNDERX_88XX
 	bool "Support ThunderX 88xx"