package/ltp-testsuite: bump version to 20210121

Add --disable-metadata configure option. Buildroot by default disable
packages' doc generation.  Also generating LTP metadata documentation
would require have host package, which could be complicated since the
LTP build system is autoconf but not automake based.

Drop cacheflush01 patch (from this release) and rebase musl workaround
patch.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>