cmd: fastboot: Kconfig: solve config issue

When FASTBOOT is activated, only one the supported device is supported in
code at the same time
- CONFIG_FASTBOOT_FLASH_MMC_DEV
- CONFIG_FASTBOOT_FLASH_NAND_DEV

But Today the choice is not exclusive in Kconfig
and that cause Kconfig issue when :
- CONFIG_FASTBOOT, CONFIG_MMC, CONFIG_NAND are activated
- CONFIG_FASTBOOT_FLASH_MMC_DEV = 0
- CONFIG_FASTBOOT_FLASH_NAND_DEV is not activated

The patch add a choice in Kconfig to select the FLASH provider
- CONFIG_FASTBOOT_FLASH_MMC
- CONFIG_FASTBOOT_FLASH_NAND

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>