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/configs/dra7xx_hs_evm_defconfig b/configs/dra7xx_hs_evm_defconfig
index 8a0ccc9..f55fc50 100644
--- a/configs/dra7xx_hs_evm_defconfig
+++ b/configs/dra7xx_hs_evm_defconfig
@@ -31,9 +31,8 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_FASTBOOT=y
 CONFIG_USB_FUNCTION_FASTBOOT=y
-CONFIG_ANDROID_BOOT_IMAGE=y
 CONFIG_FASTBOOT_BUF_ADDR=0x82000000
-CONFIG_FASTBOOT_BUF_SIZE=0x2f000000
+CONFIG_FASTBOOT_BUF_SIZE=0x2F000000
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=1
 # CONFIG_CMD_IMLS is not set