linuxptp: fix and simplify recipe, drop dependency on kernel

* Build script tries to locate the correct /usr/include/linux/net_tstamp.h just
to see if it contains definition for HWTSTAMP_TX_ONESTEP_SYNC. But that header
file hasn't changed in at least 5 years. The result of the check is to pass
additional defines to the build, which can be passed via EXTRA_OEMAKE.

* The sed command on makefile wasn't even working, plus there's no AR, prefix
is not used and better way is to use EXTRA_CFLAGS and EXTRA_LDFLAGS anyway.

* Neither autotools nor pkgconfig are use during the build, don't inherit them.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>