own-mirrors.bbclass: allow other settings of PREMIRRORS have effect

If we inherit own-mirrors.bbclass, other settings of PREMIRRORS in
our project would have no effect.

This patch modifies the setting in own-mirrors.bbclass to allow other
settings of PREMIRRORS to still have effect.

(From OE-Core rev: a3e9e7a38b3d9caed49380ebc05651a97f613c7c)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/own-mirrors.bbclass

@@ -1,13 +1,13 @@
-PREMIRRORS() {
-cvs://.*/.*     ${SOURCE_MIRROR_URL}
-svn://.*/.*     ${SOURCE_MIRROR_URL}
-git://.*/.*     ${SOURCE_MIRROR_URL}
-gitsm://.*/.*   ${SOURCE_MIRROR_URL}
-hg://.*/.*      ${SOURCE_MIRROR_URL}
-bzr://.*/.*     ${SOURCE_MIRROR_URL}
-p4://.*/.*      ${SOURCE_MIRROR_URL}
-osc://.*/.*     ${SOURCE_MIRROR_URL}
-https?$://.*/.* ${SOURCE_MIRROR_URL}
-ftp://.*/.*     ${SOURCE_MIRROR_URL}
-npm://.*/?.*    ${SOURCE_MIRROR_URL}
-}
+PREMIRRORS_prepend = " \
+cvs://.*/.*     ${SOURCE_MIRROR_URL} \n \
+svn://.*/.*     ${SOURCE_MIRROR_URL} \n \
+git://.*/.*     ${SOURCE_MIRROR_URL} \n \
+gitsm://.*/.*   ${SOURCE_MIRROR_URL} \n \
+hg://.*/.*      ${SOURCE_MIRROR_URL} \n \
+bzr://.*/.*     ${SOURCE_MIRROR_URL} \n \
+p4://.*/.*      ${SOURCE_MIRROR_URL} \n \
+osc://.*/.*     ${SOURCE_MIRROR_URL} \n \
+https?$://.*/.* ${SOURCE_MIRROR_URL} \n \
+ftp://.*/.*     ${SOURCE_MIRROR_URL} \n \
+npm://.*/?.*    ${SOURCE_MIRROR_URL} \n \
+"