mdadm: don't inherit autotools

mdadm: don't inherit autotools

mdadm doesn't use autotools at all:
- do_configure does nothing as it can't find configure.ac or configure
- do_compile is overridden to pass SYSROOT
- do_install is overridden but still calls autotools_do_install (which
  is just 'make install DESTDIR=$D') and then appended.

Clean this up by passing SYSROOT and STRIP in EXTRA_OEMAKE, removing the
now-obsolete do_compile(), and merging the do_install()s.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>