yate: fix mysql detection

yate: fix mysql detection

When configure is called with '--with-mysql="$(STAGING_DIR)/usr"' it tries
to detect mysql manually instead of using mysql_config. This doesn't always
work:

<snip>
checking for MySQL in /var/lib/buildbot/slaves/slave-lede-builds4/i386_pentium4/build/sdk/staging_dir/target-i386_pentium4_musl/usr... no
<snip>

This commit changes the configure argument to '--with-mysql=yes/no'. When
'yes' is selected configure uses mysql_config and then the detection works
reliably.

<snip>
checking for MySQL using mysql_config... yes 10.1.32
checking whether MYSQL_OPT_RECONNECT is declared... yes
checking whether MYSQL_OPT_READ_TIMEOUT is declared... yes
checking whether MYSQL_OPT_WRITE_TIMEOUT is declared... yes
checking whether mysql_set_character_set is declared... yes
<snip>

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>