board: am335x: Set serial# variable

serial# variable is needed to show the device correctly in "fastboot
devices" output. It's useful when we have several devices (in fastboot
mode) connected to single host and want to choose which one to flash.

We can't use omap_die_id_serial() for this, because AM335x lacks
DIE_ID, as can be seen from AM335x TRM. Let's do next:
 - reuse board_serial variable (obtained from EEPROM in
   set_board_info_env() function) to set serial#
 - if board_serial is "unknown", reuse ethaddr variable to set serial#

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>