Fix bootloader uninstallation on Gigabeat F / X.
The "file" bootloader installation method usually can uninstall the bootloader.
Some upgrade methods (like the Iaudio players) remove the bootloader file
placed on the player after upgrading so those can't uninstall the bootloader.
Instead of rejecting uninstallation for all players using this method check the
current bootloader and only display an error if no suitable bootloader is
found (which would happen in the Iaudio case).
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30070 a1c6a512-1295-4272-9138-f99709370657