tools: fix cross-compiling tools when HOSTCC is overridden

Richard reported U-Boot tools issues in OpenEmbedded/Yocto project.

OE needs to be able to change the default compiler. If we pass in
HOSTCC through the make command, it overwrites all HOSTCC instances,
including ones in tools/Makefile and tools/env/Makefile, which breaks
"make cross_tools" and "make env", respectively.

Add "override" directives to avoid overriding HOSTCC instances that
really need to point to the cross-compiler.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Reviewed-by: Simon Glass <sjg@chromium.org>