fru: Fix main_board_prefix meson option

Summary:
`MAIN_BOARD_PREFIX` always seemed to be the default even when setting
the `main_board_prefix` meson option. Fix the behavior so that
`MAIN_BOARD_PREFIX` is always set to the `main_board_prefix` meson
option.

Tested:
Flashed on a platform 30 unit. Can now see that the DBus path used
is correct.
Before:
```
// In 2 separate terminals, start monitoring DBus traffic prior to
// running the bifurcation binary
root@platform30:~# pcie_bifurcation
Parsing host 0
Finished parsing host 0
Parsing host 1
Finished parsing host 1
Got bus 4, addr 83, bifurcation
Using Server3PcieBifurcation implementation
Reading FRU of 4 and with address 83
Successfully populated pcie bifurcation data

// Can see that it is referencing the default
root@platform30:~# dbus-monitor --system | grep -i containing
   object path "/xyz/openbmc_project/inventory/system/board/server3_0/containing"
   object path "/xyz/openbmc_project/inventory/system/board/server3_1/containing"
```

After:
```
root@platform30:~# pcie_bifurcation
Parsing host 0
Finished parsing host 0
Parsing host 1
Finished parsing host 1
Got bus 4, addr 83, bifurcation
Using Server3PcieBifurcation implementation
Reading FRU of 4 and with address 83
Successfully populated pcie bifurcation data

// Can see that the value has changed to the user set meson option
root@platform30:~# dbus-monitor --system | grep -i containing
   object path "/xyz/openbmc_project/inventory/system/board/HPM0/containing"
   object path "/xyz/openbmc_project/inventory/system/board/HPM1/containing"
```

Google-Bug-Id: 435526024
Change-Id: Ida2298d246de7ff3768cf42e70716776473f9d85
Signed-off-by: Kyle Nieman <kyle.nieman@fii-na.corp-partner.google.com>