pkg-autotools: add a AUTOCONF_AC_CHECK_FILE_VAL macro

When configure.ac scripts do AC_CHECK_FILE tests, they always fail in
cross-compilation contexts because it is not possible to check for
file existence during the build process. Therefore we have to preseed
the configure environment with ac_cv_file_<foo>=yes variable, <foo>
being the path of the file, where all slashes, dots or dashes have
been replaced by underscores.

In the context of the boot-wrapper-aarch64, we will have to use three
of these variables, with fairly complex paths. So instead of
replicating the logic each time, we create a
AUTOCONF_AC_CHECK_FILE_VAL macro to help defining such variables.

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