If you are using the toolchain produced by OpenWrt you must setup the environment as explained here.
For the MIPS official toolchain see here.
If building your own toolchain then note binutils-2.33.1 or earlier is required.
$ make CROSS_COMPILE=<toolchain-prefix> pistachio_marduk_defconfig
$ make CROSS_COMPILE=<toolchain-prefix>
This will generate u-boot-pistachio_<board_name>-<version>.img
eg. u-boot-pistachio_marduk-2015.10-v1.0.4.img
OR u-boot-pistachio_<board_name>-<version>-<githash>.img, if there are extra commits above the tag.
e.g. u-boot-pistachio_marduk-2015.10-v1.0.4-g75cbe38.img
See our docs.