MongoDB on Raspberry Pi --dbpath not setting
Firstly instead of installing mongodb through apt just get a tar or zip of it.

1)Extract it to your home folder or wherever you would like. 

2)Mark the mongod,mongo,mongos executable.

3)make a conf file say mongod.conf and add  

 dbpath=conf/data/db
 logpath=conf/log/mongod.log
 logappend=true
 port = 27001
 fork = true


4)Start mongod by 

 path/bin/mongod --config path/mongod.conf


"bin" because you will find mongod in bin folder after extract