mdadm: convert to generic-package

mdadm: convert to generic-package

There is absolutely no reason for mdadm.mk to use autotools-package:
this package does not have any configure script at all, and its
Makefile is not generated using automake.

Therefore, convert it to use the generic-package
infrastructure. Compared to the previous code, we are now using
$(TARGET_CONFIGURE_OPTS), which passes our CPPFLAGS. This overrides
the CPPFLAGS from mdadm's Makefile, so we repeat the only CPPFLAGS
flag passed in the Makefile, -DBINDIR.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>