Use the CURRENT_DEFN field of flags

Now the CURRENT_DEFN field of the flags halfword in the pdbs_config
object is actually used, both when printing configuration objects and
when setting a current.  It never takes any value other than 0 (_I) yet,
but this easily extends to power and resistance.

src/config.c

     }
     chprintf(chp, "\r\n");
 
-    /* Print voltages and current */
+    /* Print voltage */
     chprintf(chp, "v: %d.%02d V\r\n", PD_PDV_V(cfg->v), PD_PDV_CV(cfg->v));
-    chprintf(chp, "i: %d.%02d A\r\n", PD_PDI_A(cfg->i), PD_PDI_CA(cfg->i));
+    /* Print current-deriving setting */
+    switch (cfg->flags & PDBS_CONFIG_FLAGS_CURRENT_DEFN) {
+        case PDBS_CONFIG_FLAGS_CURRENT_DEFN_I:
+            chprintf(chp, "i: %d.%02d A\r\n", PD_PDI_A(cfg->i), PD_PDI_CA(cfg->i));
+            break;
+    }
     /* If either end of the range is non-zero, print the range */
     if (cfg->vmin != 0 || cfg->vmax != 0) {
         chprintf(chp, "vmin: %d.%02d V\r\n", PD_PDV_V(cfg->vmin),

src/shell.c

     if (i >= PD_MA_MIN && i <= PD_MA_MAX && endptr > argv[0]) {
         /* Convert mA to the unit used by USB PD */
         tmpcfg.i = PD_MA2PDI(i);
+        /* Set the flags to say we're storing a current */
+        tmpcfg.flags &= ~PDBS_CONFIG_FLAGS_CURRENT_DEFN;
+        tmpcfg.flags |= PDBS_CONFIG_FLAGS_CURRENT_DEFN_I;
     } else {
         chprintf(chp, "Invalid current\r\n");
         return;