buildman: Always use the full path in CROSS_COMPILE

buildman: Always use the full path in CROSS_COMPILE

The feature to set the toolchain path does not seem to be needed. It
causes problems with venv (see [1]). Let's remove it.

Add some tests while we are here.

It does not look like any docs changes are needed for this.

[1] https://patchwork.ozlabs.org/project/uboot/patch/20240621131423.2363294-6-sjg@chromium.org/

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>