classes/populate_sdk_ext: tidy up preparation log file writing
Use a variable for the log file which includes the full path; this is
not only neater but avoids us writing the first part (the output of
oe-init-build-env) to a file in another directory since we are
changing directory as part of this subshell.
(From OE-Core master rev:
001af71752a9e9aab460cbd49ed049e1eb726295)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
