kbuild: install-extmod-build: Fix when given dir outside the build dir

kbuild: install-extmod-build: Fix when given dir outside the build dir

Commit b5e395653546 ("kbuild: install-extmod-build: Fix build when
specifying KBUILD_OUTPUT") tried to address the "build" variable
expecting a relative path by using `realpath --relative-base=.`, but
this only works when the given directory is below the current directory.
`realpath --relative-to=.` will return a relative path in all cases.

Fixes: b5e395653546 ("kbuild: install-extmod-build: Fix build when specifying KBUILD_OUTPUT")
Signed-off-by: James Le Cuirot <chewi@gentoo.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251016091417.9985-1-chewi@gentoo.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>