trees: more robust check to avoid "make fetch"

do not use shorthand here. the test was failing to
produce the desired result under some circumstances,
for example when i did "./update release" i got this:

make: Entering directory '/home/lbdev/lbmk/release/20240612-62-ga6b1a6bd/libreboot-20240612-62-ga6b1a6bd_src/src/stm32-vserprog'
make: *** No rule to make target 'fetch'.  Stop.
make: Leaving directory '/home/lbdev/lbmk/release/20240612-62-ga6b1a6bd/libreboot-20240612-62-ga6b1a6bd_src/src/stm32-vserprog'
ERROR script/trees: !mk src/stm32-vserprog fetch
ERROR ./update: excmd: script/trees -f
ERROR script/roms: Unhandled non-zero exit: ./update
ERROR ./build: excmd: script/roms serprog
ERROR ./update: build_release release/20240612-62-ga6b1a6bd: stm32
ERROR ./update: can't build rom images

in the above circumstance, run_make_command was executed,
which is not the desired behaviour; rather, fetch_project_trees
or fetch_project_repo should be called, and then the script
should immediately exit. it should also exit, without downloading
anything, if a changelog file exists as in release archives.

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

script/trees

@@ -58,8 +58,10 @@ build_projects()
 {
 	[ $# -gt 0 ] && x_ ./update trees $_f $@
 
-	[ "$mode" = "fetch" ] && [ ! -f "CHANGELOG" ] && \
-		eval "fetch_project_repo; return 0;"
+	if [ "$mode" = "fetch" ]; then
+		[ -f "CHANGELOG" ] && return 0
+		fetch_project_repo; return 0
+	fi
 
 	load_project_config "$cfgsdir"
 	[ -f "$listfile" ] || listfile="" # optional on single-tree
@@ -122,8 +124,10 @@ handle_defconfig()
 handle_src_tree()
 {
 	target_dir="$cfgsdir/$target"
-	[ "$mode" = "fetch" ] && [ ! -f "CHANGELOG" ] && \
-		eval "fetch_project_trees; return 1;"
+	if [ "$mode" = "fetch" ]; then
+		[ -f "CHANGELOG" ] && return 1
+		fetch_project_trees; return 1
+	fi
 
 	load_project_config "$target_dir"
 	x_ mkdir -p "$elfdir/$target"