ArmPlatformPkg/Bds: Fix menu when no boot entry are present for deletion or update

When there are no boot entries to delete, the boot manager was prompting the
user to select an entry to delete. This fix prints a message to let the
user know that he is trying delete boot entry when there are none.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12405 6f19259b-4bc3-4df7-8a09-765794883524