Beetle BLE: Fix variable length GATT attribute

Variable length flag was lost during attribute settings,
so variable length GATT attributes should have been set
to the predefined maximum length.
This fixes issue ARMmbed#86.

Change-Id: Ia0cd236ecd903fdb9e62a21bffef57d1e63764b9

Author: gaborkertesz

features/FEATURE_BLE/targets/TARGET_ARM_SSG/TARGET_BEETLE/source/ArmGattServer.cpp

@@ -130,6 +130,9 @@ ble_error_t ArmGattServer::addService(GattService &service)
         currAtt->pLen = p_char->getValueAttribute().getLengthPtr();
         currAtt->maxLen = p_char->getValueAttribute().getMaxLength();
         currAtt->settings = ATTS_SET_WRITE_CBACK | ATTS_SET_READ_CBACK;
+        if (p_char->getValueAttribute().hasVariableLength()) {
+            currAtt->settings |= ATTS_SET_VARIABLE_LEN;
+        }
         if (p_char->getValueAttribute().getUUID().shortOrLong() == UUID::UUID_TYPE_LONG) {
             currAtt->settings |= ATTS_SET_UUID_128;
         }