Makefile: fix newline escaping for CONFIG_DEFAULT_ENV_FILE

I wanted this to be compatible with mkenvimage, including the ability
to embed newlines in variables by escaping them. But I failed to check
that it works more than once.

Fixes: f3d8f7dd (Allow providing default environment from file)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>