syslinux: clean old object and dependency files before do_install

syslinux: clean old object and dependency files before do_install

Remove all previous .o and .d files from OBJ directories before
running do_install. This prevents stale build artifacts from
interfering with the installation of new binaries and modules,
especially after GCC upgrades that can invalidate existing dependency
files, such as:

make[4]: *** No rule to make target
'/build/tmp-glibc/work/corei7-64-wrs-linux/syslinux/6.04-pre2-r1/recipe-sysroot-native/
usr/lib/x86_64-wrs-linux/gcc/x86_64-wrs-linux/12.4.0/include/stdarg.h',
needed by 'zlib/adler32.o'.  Stop.

Signed-off-by: Haixiao Yan <haixiao.yan.cn@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>