If you are using the toolchain produced by OpenWrt you must setup the environment as explained here.
For the MIPS official toolchain see here.
$ 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.