ppp: Fix compilation errors in Makefile

ppp: Fix compilation errors in Makefile

This patch fixes below issues:

1. Make can't exit while compilation error occurs in subdir for plugins building.

2. If build ppp with newer kernel (3.10.10), it will pick 'if_pppox.h' from sysroot-dir and
   'if_pppol2tp.h' from its own source dir, this cause below build errors:

        bitbake_build/tmp/sysroots/intel-x86-64/usr/include/linux/if_pppox.h:84:26:
        error: field 'pppol2tp' has incomplete type
          struct pppol2tpin6_addr pppol2tp;
                                  ^
        bitbake_build/tmp/sysroots/intel-x86-64/usr/include/linux/if_pppox.h:99:28:
        error: field 'pppol2tp' has incomplete type
          struct pppol2tpv3in6_addr pppol2tp;
                                    ^

The 'sysroot-dir/if_pppox.h' enabled ipv6 support but the 'source-dir/if_pppol2tp.h' lost
related structure definitions, we should use both header files from sysroots to fix this
build failure.

(From OE-Core rev: b536824ea64b8d6729b830738bce637fc815e832)

Signed-off-by: Lu Chong <Chong.Lu@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>