[fb] Add default hard-coded font

For the fb driver to have a font before loading from disk is available,
create a hard-coded font as a byte array.

To create this, added a new scripts/psf_to_cpp.py which also refactored
out much of scripts/parse_font.py into a new shared module
scripts/fontpsf.py.

scripts/fontpsf.py

@@ -0,0 +1,75 @@
+_MAGIC = (0x72, 0xb5, 0x4a, 0x86)
+_VERSION = 0
+
+class PSF2:
+    from collections import namedtuple
+    Header = namedtuple("PSF2Header",
+        ["version", "offset", "flags", "count", "charsize", "height", "width"])
+
+    def __init__(self, filename, header, data):
+        self.__filename = filename
+        self.__header = header
+        self.__data = data
+
+    data = property(lambda self: self.__data)
+    header = property(lambda self: self.__header)
+    count = property(lambda self: self.__header.count)
+    charsize = property(lambda self: self.__header.charsize)
+    dimension = property(lambda self: (self.__header.width, self.__header.height))
+    filename = property(lambda self: self.__filename)
+
+    @classmethod
+    def load(cls, filename):
+        from os.path import basename
+        from struct import unpack_from
+
+        data = open(filename, 'rb').read()
+
+        fmt = "BBBBIIIIIII"
+        values = unpack_from(fmt, data)
+
+        if values[:len(_MAGIC)] != _MAGIC:
+            raise Exception("Bad magic number in header")
+
+        header = PSF2.Header(*values[len(_MAGIC):])
+        if header.version != _VERSION:
+            raise Exception(f"Bad version {header.version} in header")
+
+        return cls(basename(filename), header, data)
+
+    class Glyph:
+        __slots__ = ['index', 'data']
+        def __init__(self, i, data):
+            self.index = i
+            self.data = data
+        def __index__(self):
+            return self.index
+        def empty(self):
+            return not bool([b for b in self.data if b != 0])
+        def description(self):
+            c = chr(self.index)
+            if c.isprintable():
+                return "Glyph {:02x}: '{}'".format(self.index, c)
+            else:
+                return "Glyph {:02x}".format(self.index)
+
+    def __getitem__(self, i):
+        c = self.__header.charsize
+        n = i * c + self.__header.offset
+        return PSF2.Glyph(i, self.__data[n:n+c])
+
+    class __iter:
+        __slots__ = ['font', 'n']
+        def __init__(self, font):
+            self.font = font
+            self.n = 0
+        def __next__(self):
+            if self.n < self.font.count:
+                glyph = self.font[self.n]
+                self.n += 1
+                return glyph
+            else:
+                raise StopIteration
+
+    def __iter__(self):
+        return PSF2.__iter(self)