Use new template gbmc-bare-metal-active@.target

The systemd target gbmc-bare-metal-active.target is being changed to a
template, in order to support multi-host systems. The LinuxBootDone IPMI
command handler starts this target, so going forward, it needs to use
the systemd target gbmc-bare-metal-active@0.target, to indicate host 0.
This IPMI command is only used on single-host machines, so we can
hard-code the host number in this case.

Tested:
Exercised the LinuxBootDone IPMI command. First, set the flags
/run/bm-drive-cleaning-done-ack.flag and /run/bm-ready.flag on the BMC,
then ran the following IPMI command on the host
$ ./ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x11
 79 2b 00 11
Confirmed that the gbmc-bare-metal-active@0.target got started.

Change-Id: Iba16c3fb378eb13f17e8d48fd3a45b8b1f1da84a
Signed-off-by: John Wedig <johnwedig@google.com>
diff --git a/handler.cpp b/handler.cpp
index 2483aec..c23fdc0 100644
--- a/handler.cpp
+++ b/handler.cpp
@@ -670,7 +670,7 @@
         std::vector<uint8_t>{});
 }
 
-static constexpr auto BARE_METAL_TARGET = "gbmc-bare-metal-active.target";
+static constexpr auto BARE_METAL_TARGET = "gbmc-bare-metal-active@0.target";
 
 void Handler::linuxBootDone() const
 {