lib: provide function hextoull()

lib: provide function hextoull()

We often convert hexadecimal strings to hextoull(). Provide a wrapper
function to simple_strtoull() that does not require specifying the radix.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>