ldlinux: Fix serial output and delete eprintf()

ldlinux: Fix serial output and delete eprintf()

Tagging __syslinux_get_serial_info() with __constructor is pretty
useless when the global variables it uses, such as SerialPort, etc,
are assigned *after* the constructor has run. This constructor made
sense when config parsing was done by the core, but parsing is now
performed by ldlinux. We need to explicitly invoke the function to
initialise __syslinux_serial_console_info once we've parsed any config
files.

eprintf.c was introduced in commit 086d698c642f ("ldlinux: Add
eprintf() to print to VGA and serial") because printf() output wasn't
appearing on the serial console. It turns out that the above
__constructor confusion was the real bug.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>