[console] Treat dead keys as producing their ASCII equivalents

Treat dead keys in target keymaps as producing the closest equivalent
ASCII character, since many of these characters are otherwise
unrepresented on the keyboard.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/genkeymap.py

@@ -54,6 +54,14 @@ class KeyType(IntEnum):
     UNKNOWN = 0xf0
 
 
+class DeadKey(IntEnum):
+    """Dead keys"""
+
+    GRAVE = 0
+    CIRCUMFLEX = 2
+    TILDE = 3
+
+
 class KeyModifiers(Flag):
     """Key modifiers"""
 
@@ -96,6 +104,13 @@ class Key:
                                            KeyType.LETTER}
     """Key types with direct ASCII values"""
 
+    DEAD_KEYS: ClassVar[Mapping[int, str]] = {
+        DeadKey.GRAVE: '`',
+        DeadKey.CIRCUMFLEX: '^',
+        DeadKey.TILDE: '~',
+    }
+    """Dead key replacement ASCII values"""
+
     @property
     def keytype(self) -> Optional[KeyType]:
         """Key type"""
@@ -112,11 +127,14 @@ class Key:
     @property
     def ascii(self) -> Optional[str]:
         """ASCII character"""
-        if self.keytype in self.ASCII_TYPES:
-            value = self.value
+        keytype = self.keytype
+        value = self.value
+        if keytype in self.ASCII_TYPES:
             char = chr(value)
             if value and char.isascii():
                 return char
+        if keytype == KeyType.DEAD:
+            return self.DEAD_KEYS.get(value)
         return None