tools: Correct python building host tools

tools: Correct python building host tools

When we have python building tools for the host it will not check HOSTXX
variables but only XX variables, for example LDFLAGS and not
HOSTLDFLAGS.

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Heiko Schocher <hs@denx.de>
Fixes: 1905c8fc711a ("build: Always build the libfdt python module")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Schocher <hs@denx.de>