mtd: fixup after CPPFLAGS addition

Fixes http://autobuild.buildroot.net/results/c55e4c9741109c66f8f66ab7e3f4f1664826056a



mtd appends to CFLAGS/CPPFLAGS, but doesn't use override - So we need
to pass flags in the environment and not on the make command line to
get the combination of our flags and mtd's.

At the same time cleanup the host build to use a similar form
(HOST_CONFIGURE_OPTS) as target build.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>