my buildroot is pretty rudimentary but i came up with:
################################################################################
#
# Nim
#
################################################################################
NIM_VERSION_MAJOR = 2.0
NIM_VERSION = $(NIM_VERSION_MAJOR).2
NIM_SOURCE = nim-$(NIM_VERSION).tar.xz
NIM_SITE = https://nim-lang.org/download
#NIM_LICENSE=?
#HOST_NIM_DEPENDENCIES =?
define HOST_NIM_BUILD_CMDS
cd $(@D); ./build.sh
cd $(@D); ./bin/nim c koch
cd $(@D); ./koch boot -d:release
cd $(@D); ./koch tools
endef
# will put configs in host/etc/nim
# libs in host/usr/lib/nim
# docs in host/usr/share/nim/doc
# data in host/usr/share/nim/data
# nimble in host/opt/nimble/pkgs/nim-2.0.2
define HOST_NIM_INSTALL_CMDS
cd $(@D); env DESTDIR=$(HOST_DIR) sh install.sh /usr/bin
endef
$(eval $(generic-package))
$(eval $(host-generic-package))