buildroot: filter out dtbs KERNELNAME target (#13785)

Do not attempt to copy arch/$(LINUX_KARCH)/boot/dtbs because these are
not real files, fixes #13785.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37107 3c298f89-4303-0410-b956-a3cf2f4a3e73
prereq-build: check for "bc" (#13767)

bc is required by the kernel to compute timeconsts files, add bc to the
list of prereq to build OpenWrt.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37108 3c298f89-4303-0410-b956-a3cf2f4a3e73