serf: fix 'ccache' builds

serf: fix 'ccache' builds

'scons' cleans the environment which breaks ccache builds because
CCACHEDIR can point to an unexpected location:

| ccache arm-linux-gnueabi-gcc ... context.c
| ccache: failed to create .../serf/1.3.8-r0/.home/.ccache (No such file or directory)

Issue is described in

  http://www.scons.org/wiki/ImportingEnvironmentSettings

and because 'bitbake' cleans environment we can pass it completely
instead of trying to enumerate needed env.

With the 'env.patch' the FULLCC variable is not needed anymore (which
would break when CC is 'ccache arm-...-gcc' and host ccache is used)
because the correct $PATH is available during scons build:

| sh: .../sysroots/x86_64-oe-linux/usr/bin/arm-linux-gnueabi/ccache: No such file or directory
| scons: *** [context.o] Error 127

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>