uboot-envtools: pass target cflags

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39629 3c298f89-4303-0410-b956-a3cf2f4a3e73
mac80211: disable libertas-sd on uml target

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39630 3c298f89-4303-0410-b956-a3cf2f4a3e73
toolchain: update build steps doc

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39631 3c298f89-4303-0410-b956-a3cf2f4a3e73
target/uml: update README

invoking uml is now simplier, not need to setup console & serial

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39632 3c298f89-4303-0410-b956-a3cf2f4a3e73