build: config: allow bool to select a module pkg

kconfig-v5.6 disallowed a bool symbol to select another symbol that
'depends on m' (i.e. can be only 'm' on 'n').  It is, in fact, an unmet
dependency to have set to 'y'.  However, openwrt depends on the previous
behavior, to be able to build a package that can be a module or built-in
depending on a bool config.  This restores the previous behavior.

Ref: https://forum.openwrt.org/t/wireless-fails-on-snapshot-r12900-kernel-5-4-on-c2600-ipq806x
Tested-by: Hannu Nyman <[email protected]>
Signed-off-by: Eneas U de Queiroz <[email protected]>
Signed-off-by: Petr Štetiar <[email protected]> [added forum ref]

scripts/config/README

@@ -22,7 +22,9 @@ OpenWrt Buildroot:
    that may cause one.
  - use pre-built *.lex.c *.tab.[ch] files by default, to avoid depending on
    flex & bison.  Rebuild/remove these files only if running make with
-   BUILD_SHIPPED_FILES defined
+   BUILD_SHIPPED_FILES defined.
+ - allow a bool symbol to select a tristate symbol that depends on m (i.e.
+   a package that should only be built as a module).
 
 For a full list of changes, see the repository at:
 https://github.com/cotequeiroz/linux/commits/openwrt/scripts/kconfig

scripts/config/symbol.c

@@ -205,7 +205,7 @@ static void sym_calc_visibility(struct symbol *sym)
 	tri = yes;
 	if (sym->dir_dep.expr)
 		tri = expr_calc_value(sym->dir_dep.expr);
-	if (tri == mod && sym_get_type(sym) == S_BOOLEAN)
+	if (tri == mod)
 		tri = yes;
 	if (sym->dir_dep.tri != tri) {
 		sym->dir_dep.tri = tri;