oracle-mysql: fix hostname path

oracle-mysql: fix hostname path

MySQL detects on the build machine where the hostname program is
located, and uses this value in a number of configuration files and
scripts that are generated and installed in the target:

output/target$ grep -r "bin/hostname" *
etc/inittab:::sysinit:/bin/hostname -F /etc/hostname
usr/share/mysql/mysql.server:  pid_file=$datadir/mysqlmanager-`/usr/bin/hostname`.pid
usr/share/mysql/mysql.server:  server_pid_file=$datadir/`/usr/bin/hostname`.pid
usr/bin/mysql_install_db:hostname=`/usr/bin/hostname`
usr/bin/mysqld_safe:    err_log=$DATADIR/`/usr/bin/hostname`.err
usr/bin/mysqld_safe:  pid_file="$DATADIR/`/usr/bin/hostname`.pid"

However, the hostname on the build machine may not necessarily be at
the same location as the hostname program on the target. Buildroot has
its hostname program (coming from Busybox) in /bin, but some Linux
distributions (such as Fedora) use /usr/bin/hostname, causing the
incorrect hostname paths above.

This commit fixes that by passing the appropriate autoconf cache
variable value.

Signed-off-by: Christopher McCrory <chrismcc@gmail.com>
[Thomas: add commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>