fs_mgr: support reading fstab file from /odm or /vendor partition

fstab contains device- and soc- specific content that should reside in
/odm or /vendor partition. This change searches the fstab.${ro.hardware}
file from /odm/etc, /vendor/etc and /, then use the first one found.

Bug: 35811655
Test: boot sailfish
Change-Id: I82f89b41a849faedb64072a7cfc52d7424e1aaa1

fs_mgr/fs_mgr_fstab.cpp

@@ -636,16 +636,21 @@ struct fstab *fs_mgr_read_fstab_with_dt(const char *fstab_path)
     return in_place_merge(fstab_dt, fstab);
 }
 
-/* combines fstab entries passed in from device tree with
- * the ones found in /fstab.<hardware>
+/*
+ * tries to load default fstab.<hardware> file from /odm/etc, /vendor/etc
+ * or /. loads the first one found and also combines fstab entries passed
+ * in from device tree.
  */
 struct fstab *fs_mgr_read_fstab_default()
 {
     std::string hw;
     std::string default_fstab;
 
     if (fs_mgr_get_boot_config("hardware", &hw)) {
-        default_fstab = FSTAB_PREFIX + hw;
+        for (const char *prefix : {"/odm/etc/fstab.","/vendor/etc/fstab.", "/fstab."}) {
+            default_fstab = prefix + hw;
+            if (access(default_fstab.c_str(), F_OK) == 0) break;
+        }
     } else {
         LWARNING << __FUNCTION__ << "(): failed to find device hardware name";
     }

fs_mgr/fs_mgr_priv.h

@@ -41,8 +41,6 @@
 #define PWARNING PLOG(WARNING) << FS_MGR_TAG
 #define PERROR   PLOG(ERROR) << FS_MGR_TAG
 
-const std::string FSTAB_PREFIX("/fstab.");
-
 __BEGIN_DECLS
 
 #define CRYPTO_TMPFS_OPTIONS "size=256m,mode=0771,uid=1000,gid=1000"