infra: do not append an empty LD_LIBRARY_PATH

When we set LD_LIBRARY_PATH when building our host tools, we append any
pre-existing value to our custom path:
        LD_LIBRARY_PATH="$(HOST_DIR)/usr/lib:$(LD_LIBRARY_PATH)"

But then if LD_LIBRARY_PATH was previously empty, we end up with an
LD_LIBRARY_PATH that ends with a colon.

Also, when we check that an existing LD_LIBRARY_PATH does not contain
CWD, we previously did not look for a zero-length prefix.

Since 'man ld.so' says of LD_LIBRARY_PATH:
    A colon-separated list of directories in which to search for ELF
    libraries at execution-time. Similar to the PATH environment
    variable.

And POSIX states about PATH:
    A zero-length prefix is a legacy feature that indicates the current
    working directory.

And bash also recognises a zero-length prefix to search in CWD:
    A zero-length (null) directory name in the value of PATH indicates
    the current directory.

We may thus end up on a system where a zero-length prefix in
LD_LIBRARY_PATH is interpreted as CWD.

Do not append the previous LD_LIBRARY_PATH if it was empty, and check
for a zero-length prefix when checking dependencies.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>