x86: rename CONFIG_STACK_SIZE

Configuration variables should have the same meaning independent of the
architecture. x86 and ARM both use CONFIG_STACK_SIZE:

* x86: U-Boot's runtime stack size during reboot
* ARM: max stack size that can be used by U-Boot

Rename the x86 configuration variable to CONFIG_STACK_SIZE_REBOOT

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>