trees: reset makeargs per target/project

it wasn't being reset before. when coreboot is being
built, i add to makeargs every time. if multiple targets
are being built, the make command would end up looking
something like:

make -C src/coreboot/default UPDATED_SUBMODULES=1 \
    UPDATED_SUBMODULES=1

(the parameter would be printed twice)

of course, this doesn't check whether that parameter is
added already in target.cfg for a given target, but that's
ok because i won't add that one in target.cfg

i baked it into the code, only when handling coreboot,
because that was easier than either putting it in makeargs
for every coreboot target.cfg, or again modifying the code to
handle that; the current solution is the cleanest.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -147,7 +147,7 @@ handle_src_tree()
 load_project_config()
 {
 	eval "$(setvars "" xarch xlang tree bootstrapargs autoconfargs \
-	    xtree tree_depend)"
+	    xtree tree_depend makeargs)"
 
 	[ -f "${1}/target.cfg" ] || return 0
 	. "${1}/target.cfg" || \