fastboot: Move fastboot to drivers/fastboot

Separate CMD_FASTBOOT from FASTBOOT and move code and configuration to
drivers/fastboot.

Switch dependencies on FASTBOOT to USB_FUNCTION_FASTBOOT as anyone who wants
FASTBOOT before this series wants USB_FUNCTION_FASTBOOT. Split
USB_FUNCTION_FASTBOOT from FASTBOOT so they retain their existing
behaviour.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/am57xx_hs_evm_defconfig b/configs/am57xx_hs_evm_defconfig
index 165c2a4..9c2aef4 100644
--- a/configs/am57xx_hs_evm_defconfig
+++ b/configs/am57xx_hs_evm_defconfig
@@ -29,7 +29,7 @@
 CONFIG_SPL_SEPARATE_BSS=y
 CONFIG_SPL_DMA_SUPPORT=y
 CONFIG_SPL_SPI_LOAD=y
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x82000000
 CONFIG_FASTBOOT_BUF_SIZE=0x2F000000
 CONFIG_FASTBOOT_USB_DEV=1