autotools-package: also handle pre-installed external toolchain in .la fixup

autotools-package: also handle pre-installed external toolchain in .la fixup

The .la fixup handling looks for paths starting with /usr and assumes
that they are missing the installation prefix (i.e. $(STAGING_DIR)). It
already handles the cases that $(STAGING_DIR) itself and $(BASE_DIR)
are under /usr, but it does not yet handle the case that a
pre-installed external toolchain is under /usr (and tracks that fact
in some .la file). For instance, if you use buildroot to generate a
toolchain with HOST_DIR=/usr/local/some_path, this problem will occur.

Fix this in the same way as $(STAGING_DIR) and $(BASE_DIR), but in
addition check that TOOLCHAIN_EXTERNAL_INSTALL_DIR is non-empty. For
internal toolchains, it is empty and the sed expression would fail.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reported-by: Carlos Soto <csotoalonso@gmail.com>
Cc: Carlos Soto <csotoalonso@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>