The flexspi defconfig uses CONFIG_ENV_IS_IN_SPI_FLASH not CONFIG_ENV_IS_IN_MMC.
So when fastboot is enabled for flexspi, the build break happens.

drivers/usb/gadget/built-in.o: In function `board_fastboot_setup':
/home/leyoen/Workspace/uboot-imx/drivers/usb/gadget/f_fastboot.c:1539: undefined reference to `mmc_get_env_dev'
drivers/usb/gadget/built-in.o: In function `_fastboot_setup_dev':
/home/leyoen/Workspace/uboot-imx/drivers/usb/gadget/f_fastboot.c:1260: undefined reference to `mmc_get_env_dev'
drivers/usb/gadget/built-in.o: In function `get_single_var':
/home/leyoen/Workspace/uboot-imx/drivers/usb/gadget/f_fastboot.c:2935: undefined reference to `mmc_get_env_dev'
drivers/usb/gadget/built-in.o: In function `bcb_rw_block':
/home/leyoen/Workspace/uboot-imx/drivers/usb/gadget/bcb.c:120: undefined reference to `mmc_get_env_dev'

Fix the issue by decoupling mmc_get_env_dev function with CONFIG_ENV_IS_IN_MMC

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 2716f9a325681737593b3a6e79f94576a35067c2)