Allow more characters in partition name links

We have several partitions with underscores in their names
which would not be properly linked in:
/dev/block/platform/msm_sdcc.1/by-name/

With this change more characters (_-.) are allowed in
partition name links. Also, any other character is
replaced with '_' so the resulting link names have the
same length as the partition name.

Change-Id: I746566c03db98b10326c755692362d2c10e528ae