Workaround GCC makeinfo build bug

The GCC 4.6.x build system breaks when run on systems with sufficiently
recent versions of texinfo.  Tweak buildroot configuration to avoid this
bug.

An equivalent workaround has been commited to buildroot (commit
62322ac).  We can drop this once we upgrade buildroot and/or GCC.

GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60961

buildrootconfig

@@ -151,7 +151,7 @@ BR2_GCC_VERSION_4_6_X=y
 # BR2_GCC_VERSION_SNAP is not set
 BR2_GCC_SUPPORTS_FINEGRAINEDMTUNE=y
 BR2_GCC_VERSION="4.6.3"
-BR2_EXTRA_GCC_CONFIG_OPTIONS=""
+BR2_EXTRA_GCC_CONFIG_OPTIONS="MAKEINFO=missing"
 # BR2_INSTALL_OBJC is not set
 # BR2_INSTALL_FORTRAN is not set
 BR2_GCC_SHARED_LIBGCC=y