Something

vriera@vriera-linux build $ grep IMAGE_FSTYPES conf/local.conf 
IMAGE_FSTYPES = "cpio.gz"

vriera@vriera-linux build $ bitbake core-image-basic
Loading cache: 100% |########################################################################################################################################################################################################| ETA:  00:00:00
Loaded 1207 entries from dependency cache.
NOTE: Resolving any missing task queue dependencies
NOTE: multiple providers are available for runtime kernel-base (linux-yocto, linux-yocto-rt)
NOTE: consider defining a PREFERRED_PROVIDER entry to match kernel-base

Build Configuration:
BB_VERSION        = "1.21.0"
BUILD_SYS         = "x86_64-linux"
NATIVELSBSTRING   = "CentOS-6.4"
TARGET_SYS        = "mipsel-poky-linux"
MACHINE           = "qemumips"
DISTRO            = "poky"
DISTRO_VERSION    = "1.5+snapshot-20131216"
TUNE_FEATURES     = "o32 fpu-hard mips32"
TARGET_FPU        = ""
meta              
meta-yocto        
meta-yocto-bsp    = "master:36fe775672d66e3d83575eb532eb6ab5109343b4"

NOTE: Preparing runqueue
NOTE: Executing SetScene Tasks
NOTE: Executing RunQueue Tasks
Currently 1 running tasks (2962 of 2964):
0: core-image-basic-1.0-r0 do_rootfs (pid 38149)
NOTE: Tasks Summary: Attempted 2964 tasks of which 2962 didn't need to be rerun and all succeeded.

vriera@vriera-linux build $ ls -l tmp/deploy/images/qemumips/*.cpio.gz
-rw-r--r--. 1 vriera ldapusers 390 Dec 16 11:35 tmp/deploy/images/qemumips/core-image-basic-qemumips-20131216113457.rootfs.cpio.gz
-rw-r--r--. 1 vriera ldapusers 399 Dec 16 11:42 tmp/deploy/images/qemumips/core-image-basic-qemumips-20131216114105.rootfs.cpio.gz
-rw-r--r--. 1 vriera ldapusers 393 Dec 16 12:05 tmp/deploy/images/qemumips/core-image-basic-qemumips-20131216120445.rootfs.cpio.gz
lrwxrwxrwx. 1 vriera ldapusers  55 Dec 16 12:05 tmp/deploy/images/qemumips/core-image-basic-qemumips.cpio.gz -> core-image-basic-qemumips-20131216120445.rootfs.cpio.gz

vriera@vriera-linux build $ cd tmp/work/qemumips-poky-linux/core-image-basic/1.0-r0/temp/
vriera@vriera-linux temp $ ls -l log.do_rootfs*
lrwxrwxrwx. 1 vriera ldapusers     19 Dec 16 12:04 log.do_rootfs -> log.do_rootfs.38149
-rw-r--r--. 1 vriera ldapusers 687118 Dec 16 11:28 log.do_rootfs.30776
-rw-r--r--. 1 vriera ldapusers 100902 Dec 13 18:50 log.do_rootfs.31441
-rw-r--r--. 1 vriera ldapusers 103848 Dec 16 11:35 log.do_rootfs.33244
-rw-r--r--. 1 vriera ldapusers 106387 Dec 16 11:42 log.do_rootfs.35558
-rw-r--r--. 1 vriera ldapusers 103848 Dec 16 12:05 log.do_rootfs.38149

>>>>> log.do_rootfs.38149 is the last one