trees: unify the execution of mkhelper commands

provide it in a new function: mkhelp()

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

script/trees

@@ -104,9 +104,8 @@ build_targets()
 		target="$x"
 		printf "'make %s', '%s', '%s'\n" "$mode" "$project" "$target"
 		x_ handle_defconfig
-		[ -n "$mode" ] || [ -z "$postmake" ] || $postmake || \
-		    $err "$project/$target: !postmake: $postmake"; :
-	done; return 0
+		mkhelp "$postmake"
+	done; :
 }
 
 handle_defconfig()
@@ -342,13 +341,13 @@ handle_makefile()
 
 run_make_command()
 {
-	[ -z "$premake" ] || [ -n "$mode" ] || $premake || $err "!$premake"
+	mkhelp "$premake"
 	$dry check_cmake "$srcdir" && [ -z "$mode" ] && \
 	    $dry check_autoconf "$srcdir"
 	$dry check_makefile "$srcdir" || return 1
 
 	$dry make -C "$srcdir" $mode -j$XBMK_THREADS $makeargs || $err "!$mode"
-	[ -z "$mkhelper" ] || [ -n "$mode" ] || $mkhelper || $err "!$mkhelper"
+	mkhelp "$mkhelper"
 
 	[ "$mode" != "clean" ] || \
 	    $dry make -C "$srcdir" $cleanargs distclean || :; :
@@ -379,6 +378,11 @@ check_makefile()
 	    [ -f "$1/GNUmakefile" ] || return 1; :
 }
 
+mkhelp()
+{
+	[ -z "$1" ] || [ -n "$mode" ] || "$1" || $err "mkhelp: !$1"; :
+}
+
 copy_elf()
 {
 	[ -f "$listfile" ] && x_ mkdir -p "$dest_dir" && while read -r f; do