If you use KCONFIG_ALLCONFIG (even with empty file) you get broken
allmodconfig/allyesconfig; CONFIG_MODULES gets turned off, with obvious
massive fallout.
Breakage had been introduced when conf_set_all_new_symbols() got used
for allmodconfig et.al.
What happens is that sym_calc_value(modules_sym) done in
conf_read_simple() sets SYMBOL_VALID on both modules_sym and MODULES.
When we get to conf_set_all_new_symbols(), we set sym->def[S_DEF_USER]
on everything, but it has no effect on sym->curr for the symbols that
already have SYMBOL_VALID - these are stuck.
Solution: use sym_clear_all_valid() in there. Note that it makes
reevaluation of modules_sym redundant - sym_clear_all_valid() will do
that itself.