Kernel 5.8-rc2 or newer added a commit
f2f02ebd8f
("kbuild: improve cc-option to clean up all temporary files") , which
causes make commands to create an object output directory. Apparently
harmless. The commit was backported to stable Kernels, 4.19.131, 5.4.50,
5.7.7.
In Flatcar and Gentoo, however, `getfilevar()` in `linux-info.eclass`
runs a make command, which creates `${M}`, an object output directory,
with a root account. As Gentoo sandbox creates everything as non-root
account, the subsequent steps like `src_unpack` fail to touch the same
directory.
Upstream Gentoo already has a fix
5a3acd443c
("linux-info.eclass: Pass M=${T} to the Linux Makefile unconditionally.").
See also https://bugs.gentoo.org/729178 .
So simply sync `linux-info.eclass` with Gentoo.
5c344dfb72