aspeed: Introduce a "bmc-console" machine option

aspeed: Introduce a "bmc-console" machine option

Most of the Aspeed machines use the UART5 device for the boot console,
and QEMU connects the first serial Chardev to this SoC device for this
purpose. See routine connect_serial_hds_to_uarts().

Nevertheless, some machines use another boot console, such as the fuji,
and commit 5d63d0c76c ("hw/arm/aspeed: Allow machine to set UART
default") introduced a SoC class attribute 'uart_default' and property
to be able to change the boot console device. It was later changed by
commit d2b3eaefb4 ("aspeed: Refactor UART init for multi-SoC machines").

The "bmc-console" machine option goes a step further and lets the user define
the UART device from the QEMU command line without introducing a new
machine definition. For instance, to use device UART3 (mapped on
/dev/ttyS2 under Linux) instead of the default UART5, one would use :

  -M ast2500-evb,bmc-console=uart3

Cc: Abhishek Singh Dagur <abhishek@drut.io>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>