X86: make the SIPI num_starts a config variable

The code to figure out how to set num_starts was
starting to get kludgy. It's a constant for a given
CPU; constants should be constant; make it a config variable.

This change includes an example of how to override it.
Build but not boot tested; drivers welcome.

Change-Id: Iddd906a707bb16251615c7b42f2bfb5a044379b4
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/3796
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>

src/arch/x86/Kconfig

@@ -48,6 +48,15 @@ config MAX_REBOOT_CNT
 	int
 	default 3
 
+# This is something you almost certainly don't want to mess with.
+# How many SIPIs do we send when starting up APs and cores?
+# The answer in 2000 or so was '2'. Nowadays, on many systems,
+# it is 1. Set a safe default here, and you can override it
+# on reasonable platforms.
+config NUM_IPI_STARTS
+	int
+	default 2
+
 # We had to rename the choice options under arch/ because otherwise
 # the options would conflict between different architectures despite
 # the if ARCH_xxx guarding the arch/xxx/Kconfig sourcing.