elf2flt: fix build when zlib is not installed on the host

For a reason that's fairly unclear to me, Peter added a '-lz' link
flag to the elf2flt.mk build in d5664ee9 ("elf2flt: fix link").

However, the zlib library may not necessarily be installed on the host
machine, so we should depend on host-zlib, and pass the appropriate
LDFLAGS. This is what this patch does.

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