ANDROID: kbuild: simplify cmd_mod

Modules with a large number of compilation units can exceed execv
argument list resulting in E2BIG (Argument list too long) error.

Fix this by replacing shell 'echo > file' into a more native
$(file op filename[,text]) option.

Bug: 175420575
Change-Id: I9bc495482f16f2c9b4e05a4cb5b2283ff0c0439d
Signed-off-by: Mahesh Kumar Kalikot Veetil <mkalikot@codeaurora.org>
Signed-off-by: Jeff Johnson <jjohnson@codeaurora.org>