fastboot: move FASTBOOT_FLASH options into Kconfig

Move FASTBOOT_MBR_NAME and FASTBOOT_GPT_NAME into Kconfig.
Add dependency on the FASTBOOT_FLASH setting (also for FASTBOOT_MBR_NAME).
Remove the now redundant GPT_ENTRY_NAME.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Steve Rae <steve.rae@raedomain.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Add FIXME about xxx_PARTITION needing to be in Kconfig]
Signed-off-by: Tom Rini <trini@konsulko.com>