config.environ: always strip slash from SYSROOT (bug 654600)

Since SYSROOT=/ interacts badly with autotools.eclass (bug 654600),
and no EAPI expects SYSROOT to have a trailing slash, always strip
the trailing slash from SYSROOT.

Bug: https://bugs.gentoo.org/654600
Fixes: a41dacf7926c ("Export SYSROOT and ESYSROOT in ebuild env in EAPI 7")
Reviewed-by: James Le Cuirot <chewi@gentoo.org>
Reviewed-by: Michał Górny <mgorny@gentoo.org>
Reviewed-by: Brian Dolbec <dolsen@gentoo.org>
1 file changed