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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index 666b632..7c35138 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -5,6 +5,7 @@
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DISTRO_DEFAULTS=y
+# CONFIG_ANDROID_BOOT_IMAGE is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y
@@ -18,8 +19,6 @@
 # CONFIG_SPL_YMODEM_SUPPORT is not set
 CONFIG_FASTBOOT=y
 CONFIG_USB_FUNCTION_FASTBOOT=y
-CONFIG_FASTBOOT_BUF_ADDR=0x81000000
-CONFIG_FASTBOOT_BUF_SIZE=0x07000000
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00080000