Allow (O=...) from file

[Jay Cliburn <jacliburn@bellsouth.net>]

Sam,

This piece of the top-level Makefile in current git causes an
out-of-tree driver Makefile to fail.

101 ifdef O
102   ifeq ("$(origin O)", "command line")
103     KBUILD_OUTPUT := $(O)
104   endif
105 endif

The out-of-tree driver Makefile contains an O=... directive that
(correctly) does _not_ specify the kernel source dir, and apparently
isn't overridden by the command line either. If in the above Makefile
snippet I change "command line" to "file", my out-of-tree make
succeeds. What do you think about allowing O= to come from a file in
addition to the command line?

Here are my attempts:

[jcliburn@osprey atl1-2.0.7-20071202]$ make
make -C /lib/modules/2.6.24-rc3/source O=/lib/modules/2.6.24-rc3/build SUBDIRS=/home/jcliburn/atl1/atl1-2.0.7-20071202 modules
make[1]: Entering directory `/home/jcliburn/kernel-work/netdev/netdev-2.6.git'
Makefile:119: *** Output directory (O=...) specifies kernel src dir.  Stop.
make[1]: Leaving directory `/home/jcliburn/kernel-work/netdev/netdev-2.6.git'
make: *** [default] Error 2

[jcliburn@osprey atl1-2.0.7-20071202]$ make O=/lib/modules/2.6.24-rc3/build
make -C /lib/modules/2.6.24-rc3/source O=/lib/modules/2.6.24-rc3/build SUBDIRS=/home/jcliburn/atl1/atl1-2.0.7-20071202 modules
make[1]: Entering directory `/home/jcliburn/kernel-work/netdev/netdev-2.6.git'
Makefile:119: *** Output directory (O=...) specifies kernel src dir.  Stop.
make[1]: Leaving directory `/home/jcliburn/kernel-work/netdev/netdev-2.6.git'
make: *** [default] Error 2

Thanks for your help.

Jay