Partially revert "treewide: remove obmc-system-mgmt feature"

The obmc-system-mgmt feature is currently used in the image to trigger
inclusion of a virtual-provider which provides a number of packages many
systems need.  Partially revert the removal of this feature so that
the outcome is:

    1. The empty obmc-phosphor-sysd package is still removed.
    2. By default the 'obmc-system-mgmt' feature is included, unless
       specifically exempted.
    3. All EVB platforms remove the 'obmc-system-mgmt' feature since
       they have no system they are managing.

This partially reverts commit 060ad3ff7fcc30aff78a9e504efee9d8fa0d4526.

Tested:

  * Built `bletchley` and confirmed `packagegroup-fb-apps-system` and
    `entity-manager` are present.

```
entity-manager armv7ahf-vfpv4d16 0.1+git0+6bf41588ab-r0
packagegroup-fb-apps-system all 1.0-r1
```
  * Built `witherspoon` and confirmed `packagegroup-op-apps-system` and
    `pdbg` are present.

```
packagegroup-op-apps-system noarch 1.0
pdbg arm1176jzs 3.3
```

  * Ran `bitbake -p` on `evb-ast2600` to confirm the undefined
    `virtual-obmc-system-mgmt` is not being included in the image.

Change-Id: I8b7804d5101cc84a2c57473b3f85672bf7767c67
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-aspeed/conf/machine/evb-ast2600.conf b/meta-aspeed/conf/machine/evb-ast2600.conf
index d3ee47b..e0aa2cd 100644
--- a/meta-aspeed/conf/machine/evb-ast2600.conf
+++ b/meta-aspeed/conf/machine/evb-ast2600.conf
@@ -4,6 +4,7 @@
 SPL_BINARY = "spl/u-boot-spl.bin"
 
 require conf/machine/include/ast2600.inc
+require conf/machine/include/obmc-evb-common.inc
 
 SERIAL_CONSOLES = "115200;ttyS4"