There are two optionsou
1. Remove all source code after building:
You can indicate bitbake to remove all source code after a build adding this line
INHERIT += "rm_work"
to your conf/local.conf file. Of course, when baking another image, ALL source code has to be fetched again and it can take a lot of time, specially on machines with low connections to the Internet. The smallest image you can build is core-image-minimal, containing the minimal set of packages to board a particular machine. The largest is fsl-image-gui (in fact, there is one bigger 'fsl-image-gui-sdk' used for developing purposes inside the target). Between these two, there are a lot of images you can choose. You can even create your own image using hob.
In case you do not want to build anything, there are some pre-built images on this site, just download the sdcard file and flash it to an SD card.
2. Select only those IMAGE_FSTYPES you need.
There is a variable named IMAGE_FSTYPES which indicates the output formats you want for the resulting root file system images, by default is has this value:
IMAGE_FSTYPES ?= "tar.bz2 ext3 sdcard"
and it is located on /sources/meta-fsl-arm/conf/machine/include/imx-base.inc machine's header file. If you want to change it, place this variable on the build/conf/local.conf file. In case you have NFS mounting, remove the 'sdcard' (and either 'ext3' or 'tar.bz2') string, resulting in
IMAGE_FSTYPES = "tar.bz2"