tools/thermal/tmon: Add cross compiling support

Default to prefixed pkg-config when crosscompiling, this matches what
other parts of the tools/ directory already do.

[dlezcano] : Reworked description

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/31302992.qZodDJZGDc@devpool47