configure: fix parsing issue on libbpf_dir option

configure: fix parsing issue on libbpf_dir option

configure is stuck in an endless loop if '--libbpf_dir' option is used
without a value:

$ ./configure --libbpf_dir
./configure: line 515: shift: 2: shift count out of range
./configure: line 515: shift: 2: shift count out of range
[...]

Fix it splitting 'shift 2' into two consecutive shifts, and making the
second one conditional to the number of remaining arguments.

A check is also provided after the while loop to verify the libbpf dir
exists; also, as LIBBPF_DIR does not have a default value, configure bails
out if the user does not specify a value after --libbpf_dir, thus avoiding
to produce an erroneous configuration.

Fixes: 7ae2585b865a ("configure: convert LIBBPF environment variables to command-line options")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David Ahern <dsahern@kernel.org>