Hi Nick,
I saw exactly the same thing.
I ran make -j1 V=s as advised by the error message and saw that the error was in i2c-exp-driver:
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make[3]: Entering directory '/root/source/feeds/onion/i2c-exp-driver'
make -C /root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver CC="mipsel-openwrt-linux-musl-gcc" CFLAGS="-Os -pipe -mno-branch-likely -mips32r2 -mtune=24kc -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -msoft-float -mips16 -minterlink-mips16 -iremap /root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver:i2c-exp-driver -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -fpic" LDFLAGS="-L/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/usr/lib -L/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/lib -L/root/source/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl-1.1.16/usr/lib -L/root/source/staging_dir/toolchain-mipsel_24kc_gcc-5.4.0_musl-1.1.16/lib -znow -zrelro" LIB="-l m" PYINC=""-I/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/usr/include/python"2.7"/""
make[4]: Entering directory '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver'
**makefile:210: *** "PYTHON_VERSION variable is not set". Stop.**
make[4]: Leaving directory '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver'
Makefile:339: recipe for target '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver/.built' failed
make[3]: *** [/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/i2c-exp-driver/.built] Error 2
make[3]: Leaving directory '/root/source/feeds/onion/i2c-exp-driver'
package/Makefile:105: recipe for target 'package/feeds/onion/i2c-exp-driver/compile' failed
make[2]: *** [package/feeds/onion/i2c-exp-driver/compile] Error 2
make[2]: Leaving directory '/root/source'
package/Makefile:101: recipe for target '/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/stamp/.package_compile' failed
make[1]: *** [/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/stamp/.package_compile] Error 2
make[1]: Leaving directory '/root/source'
/root/source/include/toplevel.mk:198: recipe for target 'world' failed
make: *** [world] Error 2
In other words, the build for i2x-exp-driver wants to know the installed Python version but this information is not available due to PYTHON_VERSION not being defined. I don't know how to fix that.
I pressed on anyway, running
~/source#: **make clean**
~/source#: **git clean -dxf**
~/source#: **git pull**
~/source#: **make**
and the build ran to completion.
HOWEVER ...
I then tried downloading and building the "cross-compile" example from [https://onion.io/2bt-c-program-cross-compiling-video-tutorial/] but it failed because the Onion/LEDE build environment does not include libugpio (or, not after my git clean, anyway).
At this point I suspect that the Onion/LEDE environment in the Docker container is broken (i2c-exp-driver does not build), and it includes code which is not part of the corresponding Git repository (e.g. libugpio).
Both of those need to be fixed. Being on the bleeding edge can be "fun" but it's very frustrating if the very basics don't work!