ibm: mboxd: Replace rainier with p10bmc

The IBM rainier machine name is being replaced with the generic p10bmc.

Tested: Built rainier and verified that the mboxd service file had 64M:
$ cat tmp/work/rainier-openbmc-linux-gnueabi/obmc-phosphor-image/1.0-r0/rootfs/lib/systemd/system/mboxd.service
..
ExecStart=/usr/bin/env mboxd --flash 64M --window-size 1M

Change-Id: I7c5bf66ac38ab0b97efa1eba7ceccba86565ed85
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/meta-ibm/recipes-phosphor/mboxd/mboxd_%.bbappend b/meta-ibm/recipes-phosphor/mboxd/mboxd_%.bbappend
index e23d8fa..23d1718 100644
--- a/meta-ibm/recipes-phosphor/mboxd/mboxd_%.bbappend
+++ b/meta-ibm/recipes-phosphor/mboxd/mboxd_%.bbappend
@@ -1,6 +1,6 @@
 MBOXD_FLASH_SIZE_ibm-ac-server = "64M"
 MBOXD_FLASH_SIZE_mihawk = "64M"
-MBOXD_FLASH_SIZE_rainier = "64M"
+MBOXD_FLASH_SIZE_p10bmc = "64M"
 
 FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"