[yocto] header file missing in populated SDK

[Hans Beckérus]

Hi. We are having some issues figuring out why one of our header files
fails to be installed properly into the SDK. The header file is
currently installed using a few lines in one of our recipe:

do_install_append() {
        install -m 0644 ${S}/foo.h
${STAGING_DIR}/${MACHINE}${includedir}/linux/foo.h
}

foo.h ends up ok in our build/tmp/sysroots, but it does not make it to the SDK.
Is our do_install_append wrong in some way, does it have to be updated
to support SDK builds? Maybe it is wrong to use ${MACHINE} here?

Thanks.
Hans