RBMC: Create active BMC target

As described in the redundant BMC design document
https://gerrit.openbmc.org/c/openbmc/docs/+/70233, a redundant BMC
system can have BMCs in the active or passive state.

To become active, the obmc-bmc-active.target target will be started.
Services that need to start then can be linked to it.

This commit introduces a new rbmc-state-management meson option and
installs the target if the option is enabled.  It defaults to enabled
but will be disabled by default in the recipe.

A future commit will add more to the target.  This commit is a first
step to help get all of the bitbake options in place.

Tested:
Target file is installed when option is enabled.

Change-Id: I173c8fc78f04c12c33d335f0ffe7e5d0161d64a1
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/target_files/meson.build b/target_files/meson.build
index 9fcc2c5..979faad 100644
--- a/target_files/meson.build
+++ b/target_files/meson.build
@@ -40,6 +40,10 @@
     'obmc-power-stop@.target',
 ]
 
+if get_option('rbmc-state-management').allowed()
+  unit_files += 'obmc-bmc-active.target'
+endif
+
 fs = import('fs')
 foreach u : unit_files
     fs.copyfile(