uclibc: enable parallel building of libraries

The use of MAKE1 for uClibc dates back 10 years:

commit 8e5fb3fb
Author: Eric Andersen <andersen@codepoet.org>
Date:   Sat Dec 11 13:01:10 2004 +0000

    Add initial BR2_JLEVEL support, with some exceptions for apps that
    have broken 'make -j' support

Since that time there were lots of improvements in uClibc that seem to allow
parallel building of libs finally.

Unfortunately uClibc tests have dependences on previously built files,
that's why tests left with MAKE1.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Anton Kolesov <akolesov@synopsys.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>