Makefile: fix stray \ warnings with grep-3.8

Makefile: fix stray \ warnings with grep-3.8

We simply grep for "/usr". So no need for "-E" or "\/". Furthermore, in
the new grep versions this creates warnings.

As written in the grep-3.8 announcement:
  Regular expressions with stray backslashes now cause warnings, as
  their unspecified behavior can lead to unexpected results.
  For example, '\a' and 'a' are not always equivalent
  <https://bugs.gnu.org/39678>.

Fixes warnings in the form of:
  grep: warning: stray \ before /

Signed-off-by: Nick Hainke <vincent@systemli.org>
(cherry picked from commit a29d3bc48c40c6a2a93ae1806bea2ac26455cdbb)
[ fix conflict error ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>