Kbuild: compatibility with make-4.1

Created by: SJLC

make 4.1 parses MAKEFLAGS later than in make 3.x, which breaks '?=' assignment on variables with builtin assignments when MAKEFLAGS adds -R flag

https://savannah.gnu.org/bugs/?45211

Switch to using normal '=' assignments, which just means that non-standard values of these variables will need to be passed on the command line instead of just exported, i.e.

won't override any more

export CC=my-special-cc ; make

this is still ok

make CC=my-special-cc

Note that plain '=' is already how the linux kernel Makefile sets these same variables, so this is more consistent with kernel Kbuild.

Signed-off-by: S. Lockwood-Childs sjl@vctlabs.com