Exposing spiffs_object_truncate() in FS API

[everslick]

SPIFFS offers an function to truncate a file to a specific size. This is very useful, for rotating logfiles without going through a temp file as this needs additional space on the filesystem. Therefor I propose to expose

s32_t spiffs_object_truncate(
spiffs_fd *fd,
u32_t new_len,
u8_t remove_object);

which is defined in spiffs_nucleus.h through the FS API.

[kristianmitk]

+1

will this be added in the near future?

[earlephilhower]

Simple enough to add to #5525

[everslick]

@earlephilhower THX! That is very much appreciated!