[uart] Allow for the existence of non-16550 UARTs

Remove the assumption that all platforms use a fixed number of 16550
UARTs identifiable by a simple numeric index.  Create an abstraction
allowing for dynamic instantiation and registration of any number of
arbitrary UART models.

The common case of the serial console on x86 uses a single fixed UART
specified at compile time.  Avoid unnecessarily dragging in the
dynamic instantiation code in this use case by allowing COMCONSOLE to
refer to a single static UART object representing the relevant port.

When selecting a UART by command-line argument (as used in the
"gdbstub serial <port>" command), allow the UART to be specified as
either a numeric index (to retain backwards compatiblity) or a
case-insensitive port name such as "COM2".

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

src/include/bits/ns16550.h

@@ -1,9 +1,9 @@
-#ifndef _BITS_UART_H
-#define _BITS_UART_H
+#ifndef _BITS_NS16550_H
+#define _BITS_NS16550_H
 
 /** @file
  *
- * Dummy architecture-specific UART
+ * Dummy architecture-specific 16550-compatible UART
  *
  * This file is included only if the architecture does not provide its
  * own version of this file.
@@ -12,4 +12,4 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
-#endif /* _BITS_UART_H */
+#endif /* _BITS_NS16550_H */