[Buildroot] [grub2] possible bug in grub2.mk

[David Barbion]

Hello,

I'm facing a rather strange behaviour of grub after having 
(successfully) compiled an complete image. For my need, I use grub2 as 
bootloader.

I decided to set optimization level to 3 (-O3) for gcc.

Grub2 compiles finely but when I try to boot on a legacy BIOS, it 
displays "GRUB loading..." and then reboot infinitely...

After 2 days of tests and tries, I figured out that -O3 cause something 
not nice for grub. Indeed, compiling with -O2 solves the issue.

Regarding buildroot, I tried to force -O2 only for grub and the mk is 
done like:

[...]
GRUB2_CONF_ENV = \
     CPP="$(TARGET_CC) -E" \
     TARGET_CC="$(TARGET_CC)" \
     TARGET_CFLAGS="$(TARGET_CFLAGS)" \
     TARGET_CPPFLAGS="$(TARGET_CPPFLAGS) -fno-stack-protector" \
     TARGET_LDFLAGS="$(TARGET_LDFLAGS)" \
     TARGET_NM="$(TARGET_NM)" \
     TARGET_OBJCOPY="$(TARGET_OBJCOPY)" \
     TARGET_STRIP="$(TARGET_CROSS)strip"
[...]

In fact, envvars are passed to configure script, but instead of the 
standard CC, CFLAGS, CPPFLAGS and LDFLAGS, there are prefixed with TARGET_.
I replaced TARGET_CFLAGS and co without the TARGET_ prefix, then added 
the -O2 flag and grub2 now works, as:

[...]
GRUB2_CONF_ENV = \
     CPP="$(TARGET_CC) -E" \
     TARGET_CC="$(TARGET_CC)" \
     CFLAGS="$(TARGET_CFLAGS) -O2" \
     CPPFLAGS="$(TARGET_CPPFLAGS) -O2 -fno-stack-protector" \
     LDFLAGS="$(TARGET_LDFLAGS) -O2" \
     TARGET_NM="$(TARGET_NM)" \
     TARGET_OBJCOPY="$(TARGET_OBJCOPY)" \
     TARGET_STRIP="$(TARGET_CROSS)strip"
[...]

David.