bitbake.conf: make libc over-ride lower priority than _forcevariable

Fix long standing quirk, _forcevariable is documented as being the
highest priority over-ride.

(From OE-Core rev: b3598b8f1ed1cdc7849e1e4f30d618e87e1bf582)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/conf/bitbake.conf

@@ -735,7 +735,8 @@ DISTRO_NAME ??= "OpenEmbedded"
 #
 # This works for  functions as well, they are really just environment variables.
 # Default OVERRIDES to make compilation fail fast in case of build system misconfiguration.
-OVERRIDES = "${TARGET_OS}:${TRANSLATED_TARGET_ARCH}:build-${BUILD_OS}:pn-${PN}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${CLASSOVERRIDE}:forcevariable"
+OVERRIDES = "${TARGET_OS}:${TRANSLATED_TARGET_ARCH}:build-${BUILD_OS}:pn-${PN}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${CLASSOVERRIDE}${LIBCOVERRIDE}:forcevariable"
+LIBCOVERRIDE ?= ""
 CLASSOVERRIDE ?= "class-target"
 DISTROOVERRIDES ?= "${@d.getVar('DISTRO') or ''}"
 MACHINEOVERRIDES ?= "${MACHINE}"

meta/conf/distro/include/tclibc-baremetal.inc

@@ -4,9 +4,7 @@
 
 LIBCEXTENSION = ""
 
-# Add baremetal libc overrides to the overrides.
 LIBCOVERRIDE = ":libc-baremetal"
-OVERRIDES .= "${LIBCOVERRIDE}"
 
 ASSUME_PROVIDED += "virtual/libc virtual/libiconv virtual/crypt"
 

meta/conf/distro/include/tclibc-glibc.inc

@@ -4,9 +4,7 @@
 
 LIBCEXTENSION = "${@['', '-gnu'][(d.getVar('ABIEXTENSION') or '') != '']}"
 
-# Add glibc overrides to the overrides for glibc.
 LIBCOVERRIDE = ":libc-glibc"
-OVERRIDES .= "${LIBCOVERRIDE}"
 
 PREFERRED_PROVIDER_virtual/libiconv ?= "glibc"
 PREFERRED_PROVIDER_virtual/nativesdk-libiconv ?= "nativesdk-glibc"

meta/conf/distro/include/tclibc-musl.inc

@@ -4,9 +4,7 @@
 
 LIBCEXTENSION = "-musl"
 
-# Add musl libc overrides to the overrides.
 LIBCOVERRIDE = ":libc-musl"
-OVERRIDES .= "${LIBCOVERRIDE}"
 
 PREFERRED_PROVIDER_virtual/libc ?= "musl"
 PREFERRED_PROVIDER_virtual/libiconv ?= "musl"