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It is needed to add placeholders for this function because, as this is
not a `plat_xxx()` function, there aren't weak definitions of it in any
file.

If `console_flush()` is used and there isn't an implementation of
`console_core_flush()` in any file, the compilation will fail.

Change-Id: I50eb56d085c4c9fbc85d40c343e86af6412f3020
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>