bootstrap/7087: sub-make FLAG propagation fails if flag ends with '='

[DJ Delorie]

>     "`echo 'STAGE1_CFLAGS=$(STAGE1_CFLAGS)' | sed -e s/.*=$$/XFOO=/`"
> 
>     sed -e s'/[^=][^=]*=$$/XFOO=/'

I wonder if it would be better to just say

	's/^STAGE1_CFLAGS=$$/XFOO=/'

and avoid the wildcards completely.