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 -durN glibc-2.7.orig/Makerules glibc-2.7/Makerules

 --- glibc-2.7.orig/Makerules	2007-08-26 04:18:03.000000000 +0200

 +++ glibc-2.7/Makerules	2008-07-28 15:17:27.000000000 +0200

 @@ -1203,6 +1203,9 @@

  # The include magic above causes those files to use this variable for flags.

  CPPFLAGS-nonlib = -DNOT_IN_libc=1

 +ifneq ($(OBJDUMP_FOR_HOST),)

 +OBJDUMP = $(OBJDUMP_FOR_HOST)

 +endif

  ifeq ($(versioning),yes)

  # Generate normalized lists of symbols, versions, and data sizes.