package/memtester: fix compile and link flags

The memtester build system does not use CFLAGS/LDFLAGS variables.
Everything should be written to conf-cc and conf-ld.

Use '%' as sed expression delimiter because comma might appear in
LDFLAGS.

Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>