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>
diff --git a/README b/README
index 3ea4d01..f29bf50 100644
--- a/README
+++ b/README
@@ -1681,7 +1681,7 @@
 		to generate and write the Backup GUID Partition Table.)
 		This occurs when the specified "partition name" on the
 		"fastboot flash" command line matches this value.
-		Default is GPT_ENTRY_NAME (currently "gpt") if undefined.
+		The default is "gpt" if undefined.
 
 		CONFIG_FASTBOOT_MBR_NAME
 		The fastboot "flash" command supports writing the downloaded