altmbr: cap at 439 bytes so the partition select byte follows

altmbr: cap at 439 bytes so the partition select byte follows

Cap altmbr at 439 bytes, so that the partition select byte is not part
of the file.  This means that:

a) updating the altmbr doesn't clobber the configuration;
b) it is easier to simply concatenate the select byte to the file.

This also matches gptmbr behavior.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>