binman: Handle tool paths containing '~' correctly

At present if CROSS_COMPILE contains a tilde, such as
~/.buildman-toolchains/gcc-7.3.0-nolibc/i386-linux/bin/i386-linux-gcc
then binman gives a confusing error:

   binman: Error 255 running '~/..buildman-toolchains/gcc-7.3.0- ...

Fix this by expanding it out before running the tool.

Signed-off-by: Simon Glass <sjg@chromium.org>