Print out command line arguments for pkbootimage after splitting image

[philippelatulippe]

Thanks for writing this useful program. It came in really handy when I wanted to make some small changes to my Nexus 4 boot image but found that most splitting scripts don't take in account the Nexus' non-standard offsets in the header.

To save time when repackaging the image, I added some code that makes spbootimage print out the command line to use with pkbootimage. The only extra change I had to make was to write_chunk, which I made accept arbitrary output file names instead of concatenating the name itself.

I tested it with my Nexus 4, and the repackaged boot.img came out identical as the input.