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

--- glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h	2007-08-14 06:21:09.000000000 +0300

+++ glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-23 07:05:20.000000000 +0300

@@ -454,8 +454,7 @@

    and on Alpha just after 2.6.22-rc1.  */

 #if __LINUX_KERNEL_VERSION >= 0x020611 \

     && ((!defined __sh__ && !defined __alpha__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__) \

-	|| (__LINUX_KERNEL_VERSION >= 0x020617 && defined __alpha__))

+	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__))

 # define __ASSUME_ATFCTS	1

 #endif