PNBLACKLIST: use weak assignments

* this makes it easier to unblacklist it from local.conf which
  is parsed before the recipes

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

meta-multimedia/recipes-mediacentre/xbmc/xbmc_git.bb

@@ -10,7 +10,7 @@ DEPENDS = "libusb1 libcec libplist expat yajl gperf-native libxmu fribidi mpeg2d
 SRCREV = "82388d55dae79cbb2e486e307e23202e76a43efa"
 
 # multiple issues
-PNBLACKLIST[xbmc] = "/usr/include/c++/ctime:70:11: error: '::gmtime' has not been declared"
+PNBLACKLIST[xbmc] ?= "/usr/include/c++/ctime:70:11: error: '::gmtime' has not been declared"
 
 PV = "11.0+gitr${SRCPV}"
 PR = "r14"

meta-multimedia/recipes-multimedia/coriander/coriander_2.0.2.bb

@@ -5,7 +5,7 @@ LICENSE = "GPL-3.0"
 LIC_FILES_CHKSUM = "file://COPYING;md5=d32239bcb673463ab874e80d47fae504"
 DEPENDS = "gtk+ libgnomeui libraw1394 libdc1394 libxv tiff"
 
-PNBLACKLIST[coriander] = "BROKEN: fails to use SDL probably because libsdl-config was removed, error: unknown type name 'SDL_Overlay'"
+PNBLACKLIST[coriander] ?= "BROKEN: fails to use SDL probably because libsdl-config was removed, error: unknown type name 'SDL_Overlay'"
 
 SRC_URI = "${SOURCEFORGE_MIRROR}/project/${BPN}/coriander-2/${PV}/${BP}.tar.gz \
            file://cross-compile.patch \

meta-multimedia/recipes-multimedia/dleyna/renderer-service-upnp_0.3.0.bb

@@ -5,7 +5,7 @@ LICENSE = "LGPLv2.1"
 LIC_FILES_CHKSUM = "file://COPYING;md5=4fbd65380cdd255951079008b364516c \
                     file://src/renderer-service-upnp.c;beginline=1;endline=21;md5=c25c3082684eb4ca87474b7528c6dc15"
 
-PNBLACKLIST[renderer-service-upnp] = "BROKEN: doesn't build with B!=S (trying to install rendererconsole.py from ${B} instead of ${S})"
+PNBLACKLIST[renderer-service-upnp] ?= "BROKEN: doesn't build with B!=S (trying to install rendererconsole.py from ${B} instead of ${S})"
 
 DEPENDS = "dbus glib-2.0 gssdp gupnp gupnp-av gupnp-dlna libsoup-2.4"