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This function ensures that console output is flushed, for example
before shutting down or use by another component

In line with other console APIs, console_flush() wraps
console_core_flush().

Also implement console_core_flush() for PL011.

Change-Id: I3db365065e4de04a454a5c2ce21be335a23a01e4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>