oniro-linux.conf: Make TCLIBC weak assignment

Re-allow overriding of TCLIBC via local.conf/shell env.
Upstream part has been merged into kirkstone branch:
oe-core:e30ef926e440b505e93e404c056034d86364a241

Fixes: #608

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>

meta-oniro-core/conf/distro/oniro-linux.conf

@@ -63,7 +63,7 @@ require conf/distro/include/oniro.inc
 require conf/distro/include/oniro-wic.inc
 require conf/distro/include/oniro-packageconfig.inc
 
-TCLIBC = "musl"
+TCLIBC ?= "musl"
 
 INIT_MANAGER = "systemd"