[nap] Formalise the CPU sleeping API

src/arch/i386/interface/pcbios/bios_nap.c

@@ -0,0 +1,14 @@
+#include <gpxe/nap.h>
+#include <realmode.h>
+
+/**
+ * Save power by halting the CPU until the next interrupt
+ *
+ */
+static void bios_cpu_nap ( void ) {
+	__asm__ __volatile__ ( REAL_CODE ( "sti\n\t"
+					   "hlt\n\t"
+					   "cli\n\t" ) : : );
+}
+
+PROVIDE_NAP ( pcbios, cpu_nap, bios_cpu_nap );