style: eliminate explicit boolean comparisons

style: eliminate explicit boolean comparisons

Result of running 'hg m5style --skip-all --fix-control -a' to get
rid of '== true' comparisons, plus trivial manual edits to get
rid of '== false'/'== False' comparisons.

Left a couple of explicit comparisons in where they didn't seem
unreasonable:
invalid boolean comparison in src/arch/mips/interrupts.cc:155
>>        DPRINTF(Interrupt, "Interrupts OnCpuTimerINterrupt(tc) == true\n");<<
invalid boolean comparison in src/unittest/unittest.hh:110
>>            "EXPECT_FALSE(" #expr ")", (expr) == false)<<