meta-ibm: romulus: rename phosphor-hwmon

Rename phosphor-hwmon%.bbappend to phosphor-hwmon_%.bbappend.

Tested: Built romulus and verified hwmon confs were in rootfs.
(From meta-ibm rev: 1b03bd798e93438e60d16ea7ada0917a658f798a)

Change-Id: Ic248bd7a7e17da96e813c665f76ed1767f792d9f
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>