In some cases, passing STRIP=true is not sufficient to disable striping
altogether, as some (incorrectly generated?) configure scripts will
ignore a ${STRIP} that is not a full path.

This is the case, for example, for nmap, which ends up using the host
strip command when we pass STRIP=true:

    checking for arm-buildroot-linux-gnueabihf-strip... no
    checking for strip... /usr/bin/strip
    configure: WARNING: using cross tools not prefixed with host triplet
    [--SNIP--]
    /usr/bin/install -c -c -m 755 nping /home/ymorin/dev/buildroot/O/target/usr/bin/nping
    /usr/bin/strip -x /home/ymorin/dev/buildroot/O/target/usr/bin/nping
    /usr/bin/strip: Unable to recognise the format of the input file `/home/ymorin/dev/buildroot/O/target/usr/bin/nping'

We fix that by forcing a full path to the strip sommand when it is
disabled: STRIP=/bin/true

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>