Javaisland: check post complete status before updating the pldm retimer

Summary:
The server needs to POST complete before updating the PLDM retimer in javaisland, therefore check the POST complete status before updating the PLDM retimer.

X-link: facebookexternal/openbmc.quanta#4376

Test Plan:
1. Build passed and update BMC on javaisland.
2. Update the retimer before/after POST complete.

Tested on javaisland:
```
root@bmc-oob:~# power-util slot1 cycle
Power cycling fru 1...
root@bmc-oob:~#
root@bmc-oob:~# kv get fru1_host_ready
0
root@bmc-oob:~# fw-util slot1 --update retimer /tmp/ds160pt801_08.pldm
The server must POST complete before updating the retimer
Upgrade of slot1 : retimer failed
root@bmc-oob:~#
root@bmc-oob:~# kv get fru1_host_ready
1
root@bmc-oob:~#
root@bmc-oob:~# fw-util slot1 --update retimer /tmp/ds160pt801_08.pldm
RequestUpdate Success.
PassComponentTable Success.
UpdateComponent Success.
Download offset : 0x00001340/0x0000134d, size : 0x00000020
TransferComplete.
VerifyComplete.
ApplyComplete.
ActivateFirmwareComplete.
Upgrade of slot1 : retimer succeeded
```

Reviewed By: amithash

Differential Revision: D59937538

fbshipit-source-id: 551f0be524fcf6683d40e09f0537491422ffd1d7

meta-facebook/meta-fby35/recipes-fby35/fw-util/files/bic_pldm_retimer.cpp

@@ -31,12 +31,21 @@ int PldmRetimerComponent::update_internal(string image, bool force) {
   try {
     server.ready();
     uint8_t status;
+    char host_ready_value[8] = {0};
+    auto host_ready_key = fmt::format("fru{}_host_ready", slot_id);
+
     if (pal_get_server_power(slot_id, &status)) {
       throw string("Failed to get server power");
     }
     if (status != SERVER_POWER_ON) {
       throw string("The server power is not turned on");
     }
+    if (kv_get(host_ready_key.c_str(), host_ready_value, NULL, 0)) {
+      throw string("Failed to get POST complete status");
+    }
+    if (string(host_ready_value) != "1") {
+      throw string("The server must POST complete before updating the retimer");
+    }
   } catch (const string& e) {
     cerr << e << endl;
     return FW_STATUS_FAILURE;