Calling `load_cfg BR2_EXTERNAL_INFIX_PATH` from post-image.sh caused
warnings due to multiple hits in the .config file:
post-image.sh: 12: /tmp/tmp.5a3xhQQVc8: BR2_EXTERNAL_INFIX_PATH: not found
post-image.sh: 13: /tmp/tmp.5a3xhQQVc8: BR2_EXTERNAL_INFIX_PATH: not found
Let's grep for a "key.*=" instead of "key" to drop those warnings, while
still acting as a catch-all for partial matches.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>