Use pram_fs.h header from linux kernel

This is better than track it separately in the userspace tools tree.
Makefile: override CFLAGS and CPPFLAGS variables

This will allow the user to append compiler options via command line.
For example, it may be useful if PRAM FS kernel header is not installed
under /usr/include, but is placed somewhere else, in which case the path
to it can be specified as follows:

$ make CFLAGS=-I<path-to-linux-headers>
Replace crc16 with crc32

Kernel PRAM FS module did it, so do we.