Include boot_cpu.c for romstage builds

ROMCC boards were left unmodified.

Change-Id: I3d842196b3f5b6999b6891b914036e9ffcc3cef0
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3853
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/include/cpu/x86/lapic.h

@@ -153,6 +153,10 @@ int start_cpu(struct device *cpu);
 
 #endif /* !__PRE_RAM__ */
 
+#if CONFIG_SMP
 int boot_cpu(void);
+#else
+#define boot_cpu(x) 1
+#endif
 
 #endif /* CPU_X86_LAPIC_H */