tools: moveconfig: check compilers before starting defconfig walk

tools: moveconfig: check compilers before starting defconfig walk

Since commit 25400090b1e2 ("moveconfig: Print a message for
missing compiler"), this tool parses an error message every time an
error occurs during the process in order to detect missing compiler.

Instead of that, we can look for compilers in the PATH environment
only once before starting the defconfig walk.  If a desired compiler
is missing, "make include/config/auto.conf" will apparently fail for
that architecture.  So, the tool can just skip those board, showing
"Compiler is missing.  Do nothing.".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>