bzip2: improve the packaging

Instead of doing some nasty SED tricks, use a patch that modifies the
bzip2 build system to make it a little bit nicer. Those SEDs were made
differently for the host and target packages, which is contrary to the
logic of not having any difference in the source tree between the host
build and the target build.

This commit also makes sure that bzip2.mk behaves properly in a
BR2_PREFER_STATIC_LIB=y context.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>