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.orig/sysdeps/linux-gnu/Makefile ltrace-0.5.3/sysdeps/linux-gnu/Makefile

--- ltrace-0.5.3.orig/sysdeps/linux-gnu/Makefile	2009-07-25 17:13:02.000000000 +0200

+++ ltrace-0.5.3/sysdeps/linux-gnu/Makefile	2010-01-01 22:20:45.000000000 +0100

@@ -2,6 +2,7 @@

 				-e s/i.86/i386/ \

 				-e s/sun4u/sparc64/ \

 				-e s/sparc64/sparc/ \

+				-e s/alpha.*/alpha/ \

 				-e s/arm.*/arm/ \

 				-e s/sa110/arm/ \

 				-e s/ppc64/ppc/ \