This commit adds support for changing the default console baud rate configured
within the U-Boot bootloader. That default baud rate is exposed via the value
of the U-Boot 'baudrate' environment variable. This commit removes logic that
hardcoded the console baud rate to 115200 and instead ensures that the U-Boot
'baudrate' variable is always used when constructing the Linux kernel boot
arguments used when booting Sonic.
A change is also made to rc.local to ensure that the specified baud rate is set
correctly in the serial getty service.
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