tools: Fix default target compile tools in Python tools

tools: Fix default target compile tools in Python tools

In commit 61a15059cbb6 ("binman: Use target-specific tools when
cross-compiling"), a utility function was implemented to get preferred
compilation tools using environment variables like CC and CROSS_COMPILE.
Although it intended to provide custom default tools (same as those in
the global Makefile) when no relevant variables were set (for example
using "gcc" for "cc"), it is only doing so when CROSS_COMPILE is set and
returning the literal name of the tool otherwise.

Remove the check for an empty CROSS_COMPILE, which makes the function
use it as an empty prefix to the custom defaults and return the intended
executables.

Fixes: 61a15059cbb6 ("binman: Use target-specific tools when cross-compiling")
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>