Understood

Due to option toolchain option `-Og' is the optimization for debugging[1]
how about rename debug-build to debug-optimize?

Drop DEBUG_BUILD, use DEBUG_OPTIMIZE to instead, add debug-optimize.bbclass
and debug-optimize.inc. Use variable DEBUG_OPTIMIZE to defer inherit
debug-optimize.bbclass conditionally.(Similar did in v5)

Make a config fragment core/yocto/debug-optimize to set DEBUG_OPTIMIZE ?= 1
to enable debug friendly optimize. For the recipe (such as qemu) which doesn't
work with debug optimization, set DEBUG_OPTIMIZE = "0" to disable it for a
given recipe even though config fragment core/yocto/debug-optimize is enabled

Do not encourage user to enable debug friendly optimization by setting
DEBUG_OPTIMIZE = '1' directly, because the variable override is not certainty,
which is affected by the order of configuration parsing

V7 incoming

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-Og

//Hongxu