- introduce SGIMIPS_SGIVOL variable which specifies sgivol(8) path
because nbsgivol tool is available only in TOOLDIR for sgimips and
possibly exists in a different directory from default TOOLDIR path
- use fixed (4096) value for SGIVOLHDR.size because default value
taken from <sys/bootblock.h> is 3135 (not multiples of 4) so it
doesn't match 2048bytes/sector CD images
- also pass SGIVOLHDR.size to sgivol(8) on creating volume
- rename raw iso9660 image before creating SGIVOLHDR to avoid
incomplete (non-bootable) image left on failure of sgivol(8)
Problem (i.e. sgimipscd-3.0.2.iso is not bootable) was reported
by Thierry Lacoste on port-sgimips, and fixed images are also
tested by him on IP22 (and by me on IP32).
d6ca0e8299