LICENSE move to LICENSES/ by 882e3014 causes buildbot breakage

[Hannu Nyman]

Looks like the LICENSE file move to LICENSES/ by 882e3014 causes buildbot 
breakage.

The SDK Makefile still expects to find LICENSE file as |$(TOPDIR)/LICENSE|
https://github.com/openwrt/openwrt/blob/master/target/sdk/Makefile#L133 
<https://github.com/openwrt/openwrt/blob/master/target/sdk/Makefile#L133>

    $(CP) -L \
        $(TOPDIR)/LICENSE \

In addition to the SDK Makefile, the |$(TOPDIR)/LICENSE| is also mentioned here:

https://github.com/openwrt/openwrt/blob/master/target/toolchain/Makefile#L36 
<https://github.com/openwrt/openwrt/blob/master/target/toolchain/Makefile#L36>
||

|$(CP) $(TOPDIR)/LICENSE ./files/README.TOOLCHAIN \|

|
|

All stage1 builds with this commit 882e301461 are failing.
http://buildbot.openwrt.org/master/images/one_line_per_build 
<http://buildbot.openwrt.org/master/images/one_line_per_build>

Example error log:
http://buildbot.openwrt.org/master/images/builders/zynq%2Fgeneric/builds/713/steps/images/logs/stdio 
<http://buildbot.openwrt.org/master/images/builders/zynq%2Fgeneric/builds/713/steps/images/logs/stdio>

|./convert-config.pl /builder/shared-workdir/build/.config > 
/builder/shared-workdir/build/build_dir/target-arm_cortex-a9+neon_musl_eabi/openwrt-sdk-zynq_gcc-8.4.0_musl_eabi.Linux-x86_64/Config-build.in 
cp -fpR -L \ /builder/shared-workdir/build/LICENSE \ 
/builder/shared-workdir/build/rules.mk \ ./files/Config.in \ ./files/Makefile 
\ ./files/include/prepare.mk \ ./files/README.SDK \ 
/builder/shared-workdir/build/build_dir/target-arm_cortex-a9+neon_musl_eabi/openwrt-sdk-zynq_gcc-8.4.0_musl_eabi.Linux-x86_64/ 
cp: cannot stat '/builder/shared-workdir/build/LICENSE': No such file or 
directory Makefile:88: recipe for target 
'/builder/shared-workdir/build/bin/targets/zynq/generic/openwrt-sdk-zynq_gcc-8.4.0_musl_eabi.Linux-x86_64.tar.xz' 
failed make[2]: Leaving directory '/builder/shared-workdir/build/target/sdk' |


Buildbot still built ok ce4cb8e 
<https://github.com/openwrt/openwrt/commit/ce4cb8e51d8d4b24267653250c3f7310d208bfaa>, 
but fails all builds since 36bb119 
<https://github.com/openwrt/openwrt/commit/36bb1196d16eada7dff05540dbeb74ed9f54e600> 
which is only a few commits later (including this one). This commit looks 
like the probable culprit in that commit range.