scons.bbclass: Make MAXLINELENGTH overridable

older scons do not support MAXLINELENGTH and some packages still may be
using older scons, these recipes can clear SCONS_MAXLINELENGTH in them
and get going. Set

SCONS_MAXLINELENGTH = ""

in such recipes.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>