Support reading and writing the HV_Preferred flag

This commit adds the toggle_hv_preferred command, and makes printed
configuration objects show the state of the HV_Preferred flag.  The
flag still isn't used by anything else yet though.

src/config.c

     if (cfg->flags & PDBS_CONFIG_FLAGS_VAR_BAT) {
         chprintf(chp, "Var/Bat ");
     }
+    if (cfg->flags & PDBS_CONFIG_FLAGS_HV_PREFERRED) {
+        chprintf(chp, "HV_Preferred ");
+    }
     chprintf(chp, "\r\n");
 
     /* Print voltages and current */

src/shell.c

     tmpcfg.flags ^= PDBS_CONFIG_FLAGS_GIVEBACK;
 }
 
+static void cmd_toggle_hv_preferred(BaseSequentialStream *chp, int argc, char *argv[])
+{
+    (void) argv;
+    if (argc > 0) {
+        chprintf(chp, "Usage: toggle_hv_preferred\r\n");
+        return;
+    }
+
+    /* Toggle the HV_Preferred flag */
+    tmpcfg.flags ^= PDBS_CONFIG_FLAGS_HV_PREFERRED;
+}
+
 static void cmd_set_v(BaseSequentialStream *chp, int argc, char *argv[])
 {
     if (argc != 1) {
     {"get_tmpcfg", cmd_get_tmpcfg, "Print the configuration buffer"},
     {"clear_flags", cmd_clear_flags, "Clear all flags"},
     {"toggle_giveback", cmd_toggle_giveback, "Toggle the GiveBack flag"},
+    {"toggle_hv_preferred", cmd_toggle_hv_preferred, "Toggle the HV_Preferred flag"},
     /* TODO {"toggle_var_bat", cmd_toggle_var_bat, "Toggle the Var/Bat flag"},*/
     {"set_v", cmd_set_v, "Set the voltage in millivolts"},
     {"set_i", cmd_set_i, "Set the current in milliamps"},