Add new flag macros, clear flags with discretion

Firmware 1.2.0 will have more configuration flags than previous
versions.  This commit adds macros for those flags.  There's also a
field added to the flags halfword that says whether a current, power, or
resistance is configured, so this commit makes the clear_flags command
not modify that field.

src/config.h

  * PD Buddy Sink configuration structure
  */
 struct pdbs_config {
+    /* Status halfword, used to indicate which config objects can be written to
+     * and which one is valid. */
     uint16_t status;
+    /* Flags halfword for miscellaneous small fields. */
     uint16_t flags;
+    /* Preferred voltage. */
     uint16_t v;
-    uint16_t i;
+    /* Union for specifying how much current to request. */
+    union {
+        /* Required current. */
+        uint16_t i;
+        /* Required power. */
+        uint16_t p;
+        /* Value of resistive load. */
+        uint16_t r;
+    };
+    /* Lower end of voltage range. */
     uint16_t vmin;
+    /* Upper end of voltage range. */
     uint16_t vmax;
+    /* Extra bytes reserved for future use. */
     uint16_t _reserved[2];
 } __attribute__((packed));
 
 
 /* Flags for configuration structures. */
 /* GiveBack supported */
-#define PDBS_CONFIG_FLAGS_GIVEBACK 0x0001
+#define PDBS_CONFIG_FLAGS_GIVEBACK (1 << 0)
 /* Variable and battery PDOs preferred (FIXME: not implemented) */
-#define PDBS_CONFIG_FLAGS_VAR_BAT 0x0002
+#define PDBS_CONFIG_FLAGS_VAR_BAT (1 << 1)
+/* High voltages preferred */
+#define PDBS_CONFIG_FLAGS_HV_PREFERRED (1 << 2)
+/* Current definition type */
+#define PDBS_CONFIG_FLAGS_CURRENT_DEFN_SHIFT 3
+#define PDBS_CONFIG_FLAGS_CURRENT_DEFN (0x3 << PDBS_CONFIG_FLAGS_CURRENT_DEFN_SHIFT)
+#define PDBS_CONFIG_FLAGS_CURRENT_DEFN_I (0 << PDBS_CONFIG_FLAGS_CURRENT_DEFN_SHIFT)
+#define PDBS_CONFIG_FLAGS_CURRENT_DEFN_P (1 << PDBS_CONFIG_FLAGS_CURRENT_DEFN_SHIFT)
+#define PDBS_CONFIG_FLAGS_CURRENT_DEFN_R (2 << PDBS_CONFIG_FLAGS_CURRENT_DEFN_SHIFT)
 
 
 /* Flash configuration array */

src/shell.c

         return;
     }
 
-    /* Clear all flags */
-    tmpcfg.flags = 0;
+    /* Clear all flags that can be toggled with toggle_* commands */
+    tmpcfg.flags &= ~PDBS_CONFIG_FLAGS_GIVEBACK
+        & ~PDBS_CONFIG_FLAGS_VAR_BAT
+        & PDBS_CONFIG_FLAGS_HV_PREFERRED;
 }
 
 static void cmd_toggle_giveback(BaseSequentialStream *chp, int argc, char *argv[])