fru: Remove '_' from Entity Manager main path

Summary:
Not all platforms follow the format of <Platform Name>_<Host Index>.
For platform 30, the main board path format is HPM<Host Index>.

Tested:
Flashed on a platform 30 Pre-EVT unit. Could successfully provide the
option to meson.

Google-Bug-Id: 435526024
Change-Id: Ib897540f18bae551bf6f59109344a738449953a1
Signed-off-by: Kyle Nieman <kyle.nieman@fii-na.corp-partner.google.com>