perf: Ensure we use CFLAGS and LDFLAGS settings from kernel build system

perf: Ensure we use CFLAGS and LDFLAGS settings from kernel build system

The kernel build system does the right thing here and we should stop
overriding it. This code has been added based on a change from
'meta-metro' layer, revision 9d698004137c1a888d40d6a4808d94afa22387e7,
without any information about what problem it fixes so I am reverting
it.

Using the CFLAGS and LDFLAGS makes it impossible for kernel build
system to append to it, thus making the build fail in various ways as:

|     CC /.../perf/1.0-r8/perf-1.0/perf.o
| In file included from builtin.h:4:0,
|                  from perf.c:9:
| util/util.h:74:24: fatal error: lk/debugfs.h: No such file or directory
|  #include <lk/debugfs.h>
|                         ^
| compilation terminated.

The unset is done in do_compile and do_install otherwise it /rebuild/
perf as it detects the compiler options has change.

(From OE-Core rev: 2e90f8846db0f3ed99a175befff9ec67fe12bc4e)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>