perf: Fix parsing error

perf: Fix parsing error

Due to the use of ${@...} the code will try and expand this when
performing the initial parsing. If the sysroot doesn't exist with
an existing kernel, this will fail at parsing time.

Sinec we're already in python, just remove the ${@....} wrapping
and then we execute at do_package time which is what we want.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>