The '?=' will not assign value if the target make variable is overriden via command-line or is set in environment variable. On other hand, '=' will not assign value only if variable is overriden via command-line parameter. It is quite common to have CC, AS, CPP, LD, etc to be set as environment variables pointing to native compiler and binutils. If '-rR' option is not set in MAKEFLAGS then this results in compile error because '?=' will use the native compiler and binutils. If '-rR' option is set in MAKEFLAGS then this again results in compile error because CC, AS, CPP, etc are set to empty strings which causes '?=' to use empty string as compiler and binutils. To handle this, we use '?=' only when CROSS_COMPILE is not defined and we use '=' when CROSS_COMPILE is defined. Signed-off-by: Anup Patel <anup.patel@wdc.com>