netopt: add NETOPT_BATMON

Many radios have a built-in battery monitor.
Add a netopt to configure it.

sys/include/net/netopt.h

@@ -767,6 +767,16 @@ typedef enum {
      */
     NETOPT_RSSI,
 
+    /**
+     * @brief (uint16_t) Set the battery monitor voltage (in mV).
+     *
+     * When set, a @ref SYS_BUS_POWER_EVENT_LOW_VOLTAGE event is generated
+     * on the SYS_BUS_POWER bus if the supply voltage falls below the set value.
+     *
+     * Set to 0 to disable battery monitoring.
+     */
+    NETOPT_BATMON,
+
     /**
      * @brief   (array of byte array) get link layer multicast groups as array
      *          of byte arrays (length of each byte array corresponds to the

sys/net/crosslayer/netopt/netopt.c

@@ -126,6 +126,7 @@ static const char *_netopt_strmap[] = {
     [NETOPT_NUM_GATEWAYS]          = "NETOPT_NUM_GATEWAYS",
     [NETOPT_LINK_CHECK]            = "NETOPT_LINK_CHECK",
     [NETOPT_RSSI]                  = "NETOPT_RSSI",
+    [NETOPT_BATMON]                = "NETOPT_BATMON",
     [NETOPT_L2_GROUP]              = "NETOPT_L2_GROUP",
     [NETOPT_L2_GROUP_LEAVE]        = "NETOPT_L2_GROUP_LEAVE",
     [NETOPT_NUMOF]                 = "NETOPT_NUMOF",

tests/unittests/Makefile.ci

@@ -43,8 +43,10 @@ BOARD_INSUFFICIENT_MEMORY := \
     lsn50 \
     maple-mini \
     mega-xplained \
+    mcb2388 \
     microbit \
     microduino-corerf \
+    msba2 \
     msb-430 \
     msb-430h \
     nrf51dk \