host-build: fix stampfile name substitution per installed binaries

host-build: fix stampfile name substitution per installed binaries

A funny bug was discovered where if the buildroot's path
has the name of the build target within it, it will also be substituted
along with the stampfile's name for each program,
causing an attempt to touch a file in a directory that doesn't exist.

...
...
touch: cannot touch '/Volumes/touch/openwrt/staging_dir/host/stamp/.touch_installed': No such file or directory
touch: cannot touch '/Volumes/ln/openwrt/staging_dir/host/stamp/.ln_installed': No such file or directory
touch: cannot touch '/Volumes/chown/openwrt/staging_dir/host/stamp/.chown_installed': No such file or directory
make[2]: *** [Makefile:50: /Volumes/coreutils/openwrt/staging_dir/host/stamp/.coreutils_installed] Error 1
...
...

Split up the path with $(dir) and $(notdir) before substitution to fix
the syntax.

Reported-by: Georgi Valkov <gvalkov@gmail.com>
Tested-by: Georgi Valkov <gvalkov@gmail.com> # MacOS
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>