scripts/functions: add a generic custom location infrastructure

Add a generic custom location infrastructure (inspired by the one in
kernel/linux) to allow the user to use custom tarballs or directories
for any component.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
[yann.morin.1998@free.fr: move config option, improve help text, fix API doc]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <131c163c69f9cc81d2be.1349931191@localhost.localdomain>
PatchWork-Id: 190784
Message-Id: <0bbaba9190a76ba97f72.1349931192@localhost.localdomain>
PatchWork-Id: 190785

diff -ru ltrace-0.5.3.org/configure ltrace-0.5.3/configure

--- ltrace-0.5.3.org/configure	2011-08-21 18:55:15.000000000 +0200

+++ ltrace-0.5.3/configure	2011-08-21 18:54:46.000000000 +0200

@@ -15,8 +15,12 @@

 echo $PACKAGE_VERSION

 echo -n "checking HOST_OS... "

-HOST_OS=$( uname -s )

-if [ "$HOST_OS" = "Linux" ]

+if [ -z "$HOST_OS" ] ; then

+  HOST_OS=$( uname -s )

+else

+  echo -n "using preset: "

+fi

+if [ "$HOST_OS" = "Linux" -o "$HOST_OS" = "linux" ]

 then

   HOST_OS="linux-gnu"

 fi