fixes flash on debian 8.3

[StefanScherer]

@jeanescoto Thank you. So a sudo is needed to write the file in the FAT partition of the SD card? Can you write other files with your user account into that partition? Normally this should be mounted read-writable for the current user.

[jeanescoto]

@StefanScherer

You see, An operation performed by root only sets writable permission to root

• A non-existing mnt will be created (mkdir -p ${boot}) with user permissions
• The fat partition is mounted by root (sudo mount "${dev}" "${boot}") on /tmp/mnt;
• System sets /tmp/mnt to root:root with mode 755 (rwxr-wr-w);
• Even if I change boot variable to be boot=~/mnt mounting the fat partition on~/mnt will change ~/mnt to root and 755.

In order to not use sudo a chmod or chown should be done on directory /tmp/mnt after mounted.

[StefanScherer]

Ok, as we need sudo for other commands I think this is good enough to keep it simple.