Linux 3.12 made it into Debian unstable, fetch it from there.

auto/config

@@ -35,7 +35,7 @@ $RUN_LB_CONFIG \
    --memtest none \
    --packages-lists="standard" \
    --tasks="standard" \
-   --linux-packages="linux-image-3.12-trunk" \
+   --linux-packages="linux-image-3.12-1" \
    --syslinux-menu vesamenu \
    --syslinux-splash data/splash.png \
    --syslinux-timeout 4 \

config/chroot_apt/preferences

@@ -235,68 +235,64 @@ Package: linux-base
 Pin: release o=Debian,a=testing
 Pin-Priority: 999
 
-Package: linux-compiler-3.12-x86
-Pin: release o=Debian,a=experimental
-Pin-Priority: 999
-
 Package: linux-compiler-gcc-4.8-x86
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-headers-486
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-headers-686-pae
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-headers-amd64
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-headers-3.12-trunk-common
-Pin: release o=Debian,a=experimental
+Package: linux-headers-3.12-1-common
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-headers-3.12-trunk-486
-Pin: release o=Debian,a=experimental
+Package: linux-headers-3.12-1-486
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-headers-3.12-trunk-686-pae
-Pin: release o=Debian,a=experimental
+Package: linux-headers-3.12-1-686-pae
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-headers-3.12-trunk-amd64
-Pin: release o=Debian,a=experimental
+Package: linux-headers-3.12-1-amd64
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-image-486
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-image-686-pae
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-image-amd64
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-image-3.12-trunk-486
-Pin: release o=Debian,a=experimental
+Package: linux-image-3.12-1-486
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-image-3.12-trunk-686-pae
-Pin: release o=Debian,a=experimental
+Package: linux-image-3.12-1-686-pae
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: linux-image-3.12-trunk-amd64
-Pin: release o=Debian,a=experimental
+Package: linux-image-3.12-1-amd64
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: linux-kbuild-3.12
-Pin: release o=Debian,a=experimental
+Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
 Package: laptop-mode-tools