yaffs2-utils: set EXTRA_OEMAKE as required

bitbake.conf has now removed "-e MAKEFLAGS=" as the default value of
EXTRA_OEMAKE. However, yaffs2-utils needs this option to make compile succeed.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>