uuid: get_uuid(): add an argument, instead of global variable
common: move common helpers/consts/vars into separate script
image_sender: move reserve from get_image_size_in_bs() to caller
get_uuid: add brief
image_sender: move casting to *_in_bs inside main loop (was in helper)
resize2fs: use get_fs_size_in_kb() instead of get_min_block_size_kb()

And delete get_min_block_size_kb().

Since get_min_block_size_kb() need to mount fs, which is not so good,
the better desicion is to use dumpe2fs to calculate fs size

See-also: 0535695e286aeb1cb5a9c2f6b29af4d241602de4 ("image_sender: final
fix for copying not whole block device")
resize2fs: update comment about min-size in resize2fs