fastboot: Correct dependencies in FASTBOOT_FLASH

Ensure that when selecting FASTBOOT_FLASH you end up with a buildable
configuration. Prior to this you could select NAND without MTDPARTS
and end up with an image which (surprisingly) excluded NAND.

Also fix dependencies on FASTBOOT_GPT_NAME/FASTBOOT_MBR_NAME which require
you have EFI_PARTITION/DOS_PARTITION enabled.

Delete redundant FASTBOOT_FLASH_NAND_DEV from Kconfig - it was only ever
used as a guard and the value was ignored in all cases, we're using
FASTBOOT_FLASH_NAND as the guard now.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>