log: respect logger configured before EAL init

log: respect logger configured before EAL init

Before this patch, application-specific loggers could not be
installed before rte_eal_init completed (the initialization process
called rte_openlog_stream, overwriting any previously installed
logger). This made it impossible for an application to capture the
initial log messages generated during rte_eal_init. This patch changes
initialization so that information from a previous call to
rte_openlog_stream is not lost. Specifically:
* The default log stream is now maintained separately from an
  application-specific log stream installed with rte_openlog_stream.
* rte_eal_common_log_init has been renamed to eal_log_set_default,
  since this is all it does. It no longer invokes rte_openlog_stream; it
  just updates the default stream. Also, this method now returns void,
  rather than int, since there are no errors.

This patch also removes the "early log" mechanism and cleans up the
log initialization mechanism:
* The default log stream defaults to stderr on all platforms if
  eal_log_set_default hasn't been invoked (Linux used to use stdout
  during the first part of initialization).
* Removed rte_eal_log_early_init; all of the desired functionality can
  be achieved by calling eal_log_set_default.
* Removed lib/librte_eal/bsdapp/eal/eal_log.c: it contained only one
  function, rte_eal_log_init, which is not needed or invoked for BSD.
* Removed declaration for eal_default_log_stream in rte_log.h (it's now
  private to eal_common_log.c).
* Moved call to rte_eal_log_init earlier in rte_eal_init for Linux, so
  that it starts using the preferrred log ASAP.

Signed-off-by: John Ousterhout <ouster@cs.stanford.edu>