schmoogie: fix build error due to removal of forceenv()

commit 6d014adf dropped support for the forceenv() function, but failed
to remove references to it from board/davinci/schmoogie/schmoogie.c

Replace forceenv() by setenv() and set CONFIG_ENV_OVERWRITE instead in
the board config file to allow overwriting the serial number.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Sergey Kubushyn <ksi@koi8.net>
Acked-by: Sergey Kubushyn <ksi@koi8.net>