common/console.c: use CONFIG_VAL() with PRE_CON_BUF_* variables

There is currently no support for PRE_CONSOLE_BUFFER in SPL, but if
and when that gets implemented, one would almost certainly want to use
a different address and/or size for the buffer (e.g., U-Boot proper
might specify an address in DRAM and a generous buffer, while SPL
would be much more constrained).

So a prerequisite for adding SPL_PRE_CONSOLE_BUFFER is to make the
code use SPL_-specific values. No functional change.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>