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/meson.options b/meson.options
index 0c86bcb..b076cb0 100644
--- a/meson.options
+++ b/meson.options
@@ -150,3 +150,8 @@
     value : false,
     description : 'Only allow chassis and host power on operations when BMC is Ready.'
 )
+
+option('rbmc-state-management', type: 'feature',
+    value : 'enabled',
+    description : 'Enable redundant BMC state management'
+)