init,halt: portability improvements

* make init and halt use the same RB_* constants for reboot()
* conditionalize the Linux-specific code

Inspired by init.init.diff from the Debian kFreeBSD patches at:
http://svn.debian.org/viewsvn/d-i/people/slackydeb/kfreebsd/busybox/1.14/debian

Signed-off-by: Jeremie Koenig <jk@jk.fr.eu.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

init/reboot.h

@@ -0,0 +1,31 @@
+/*
+ * Definitions related to the reboot() system call,
+ * shared between init.c and halt.c.
+ */
+
+#include <sys/reboot.h>
+
+#ifndef RB_HALT_SYSTEM
+# if defined(__linux__)
+#  define RB_HALT_SYSTEM  0xcdef0123
+#  define RB_ENABLE_CAD   0x89abcdef
+#  define RB_DISABLE_CAD  0
+#  define RB_POWER_OFF    0x4321fedc
+#  define RB_AUTOBOOT     0x01234567
+# elif defined(RB_HALT)
+#  define RB_HALT_SYSTEM  RB_HALT
+# endif
+#endif
+
+/* Stop system and switch power off if possible.  */
+#ifndef RB_POWER_OFF
+# if defined(RB_POWERDOWN)
+#  define RB_POWER_OFF  RB_POWERDOWN
+# elif defined(__linux__)
+#  define RB_POWER_OFF  0x4321fedc
+# else
+#  warning "poweroff unsupported, using halt as fallback"
+#  define RB_POWER_OFF  RB_HALT_SYSTEM
+# endif
+#endif
+