When using "make -j" we must make silentoldconfig before building
any object. Otherwise we may use the previous autoconf.h, in error
if we just changed the configuration.

This was exposed by ./MAKEALL with the properly working wr/non-wr build
for wrs: previous wrcp_defconfig left CONFIG_EXT_WR active while
building some file with wrs-nonwr_defconfig.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>