external-toolchain: better documentation, cleanup, sysroot check

 * Introduce documentation for each function of ext-tool.mk, and
   document all parameters of the functions.

 * Pass SYSROOT_DIR as argument to all functions that require it,
   instead of computing it manually everywhere

 * Use $(shell) instead of backquotes

 * Check that the SYSROOT_DIR variable is not empty, which means that
   the external toolchain doesn't support --sysroot. In that case,
   bail out with a nice error message.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>