build: add a prereq check for file (#15692)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40568 3c298f89-4303-0410-b956-a3cf2f4a3e73
build: clarify the file prereq check message

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40569 3c298f89-4303-0410-b956-a3cf2f4a3e73