package/cargo: pass appropriate library path to the linker

When linking the host cargo binary, the linker should be told to find
libraries in $(HOST_DIR)/lib, otherwise it will not work libraries
such as libhttp_parser. This was found with per-package directory
support, where the build failed with:

  = note: /usr/bin/ld: cannot find -lhttp_parser
          collect2: error: ld returned 1 exit status

In order to fix this, instead of passing -L$(HOST_DIR)/lib during the
build of Cargo, we make sure all flags in $(HOST_LDFLAGS) are passed.

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