tcsh: fix build on macos

tcsh requires host-tool (gethost) to generate tc.defs.c file.
To build this tool on macos, it is required to run ./configure
script wihout target vars/flags, but this package is not
splitted to host and target build.

This patch splits build to host and target builds to compile
host-tool(gethost) first, then compile tsch.

Due to lack of support of separate building host and target,
OpenWrt Makefile is modified to do it in the order below:
1. build host-tool (gethost)
2. build sh.err.h header for target
3. build tc.const.h header for target
4. copy gethost tool from host staging dir to target build dir
5. touch (make -t) gethost to protect it from recompiling
6. build tsch for target

It is required to do step 2 and 3 because 'make -t' generates
empty files but these files required to build tcsh

Patch 020-cross.patch has been removed due to not required
anymore (gethost build is separated and not affected by
target configure flags and vars)

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>