drivers/net/tun.c has a missing dependency on enabling the crc32
libraries in kernel config.   With tun enabled and crc32 disabled "make
bzImage" (the linking step) fails. For example:

drivers/built-in.o(.text+0x656f1): In function `add_multi':
linux-2.5cifs/drivers/net/tun.c:112: undefined reference to `crc32_le'
drivers/built-in.o(.text+0x656f9):linux-2.5cifs/drivers/net/tun.c:112:
undefined reference to `bitreverse'

Line 112:

	        int bit_nr = ether_crc(ETH_ALEN, addr) >> 26;

is a call to the ether_crc macro which maps to the bitreverse function
which is only exported if you enable:
	library functions -> CRC32 functions 
in kernel config.  The following would fix it.