[Buildroot] Is it possible to override a package version from BR2_EXTERNAL?

[Patrick Doyle]

I am asking because it appears (for the moment) that the 2016.01
host-uboot-tools mkenvimage utility does not generate an environment
file compatible with my non-2016.01 u-boot.

I will try changing UBOOT_TOOLS_VERSION to 2015.01 in uboot-tools.mk,
but that is not a long term solution -- I would rather use a pristine
buildroot source tree with by BR2_EXTERNAL customizations for kernel
_defconfig files & the like.

Or, it's possible that something else entirely is wrong with my
environment file and it has nothing to do with mkenvimage.  But I am
still curious about overriding specific package revisions on a
case-by-case basis.

--wpd