vfs_open/vfs_unlink will the file found first in any VFS device
ds1307 datasheet:
A master must signal an end of data to the slave by not generating
an acknowledge bit on the last byte that has been clocked out of
the slave.
no need to block interrupts between i2c read and writes
coding style
vfs_create will use the first VFS device that is writeable
byte order is little endian - bit7 comes last
Merge pull request #348 from eku/vfs_fix

vfs_open/vfs_unlink will use the file found first in any VFS device
Merge pull request #349 from eku/ds1337_fix

Ds1337 fix