builder takes _BL_SPACE too seriously in boot images

[wesolows]

Currently we test that an ELF boot image has a _BL_SPACE symbol and that its value is exactly equal to the amount of space occupied by (in effect) everything below the base of the BSS. Normally this is fine, but it's needlessly restrictive: if there's extra space allocated that's unused -- in sufficient quantity that it causes us to allocate unnecessary space beyond __ebss -- then we fail with an IncompatibleExecutable error. In fact this is harmless and we could test only to see whether totalsz <= symsz.