packages: use single = not == for string comparison

[MilhouseVH]

Small cosmetic cleanup to fix a few instances of == that have slipped in when the project standard is = (see #3055).

I've not bothered to fix

LibreELEC.tv/projects/Amlogic_Legacy/packages/device-trees-amlogic/package.mk

Lines 41 to 46 in 6b93ae6

	[[ "$f" == gxbb* ]] || [[ "$f" == gxl* ]] && DTB_LIST_FILTERED="$DTB_LIST_FILTERED $f"
	done
	;;
	S912)
	for f in ${DTB_LIST[@]}; do
	[[ "$f" == gxm* ]] && DTB_LIST_FILTERED="$DTB_LIST_FILTERED $f"

as this file is due for the chop soon (#3458)