check-package: fix check of file in current dir with -b

check-package: fix check of file in current dir with -b

One of the possible usages of check-package is to first cd to the
directory that contains the files to test (e.g. a package directory) and
then call the script passing the files in the current dir.
It already works when used for intree files, but for files in a
br2-external it throws an exception because some check functions (from
utils/checkpackagelib/lib_*.py) do need the name of the file being
processed and assume there will be a slash before the name.

Fix all check functions that assume that the full filename being checked
contains a slash. Do not use regexps to extract the filename, use
os.path functions instead.

Notice RemoveDefaultPackageSourceVariable and TypoInPackageVariable lead
to an exception in this case, but ApplyOrder instead generates a false
warning.

Fixes bug #11271.

Reported-by: Vitaliy Lotorev <lotorev@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Vitaliy Lotorev <lotorev@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>