Makefile.rules: Allow specifying HOST_LDFLAGS for cross-compilation
Makefile.rules: Allow specifying HOST_LDFLAGS for cross-compilation

Add HOST_LDFLAGS to cmd_c_to_host to allow for cross-compilation of
utils such as 'ectool'.

BRANCH=master
BUG=none
TEST=Cross compile using arm-oe-linux-gnueabi with corresponding
linkerflags and run resulting binary on armv7 device

Change-Id: Iebdfa92239eb5f245d4ad3623cbdceafd82d1675
Signed-off-by: Moritz Fischer <16c4312f096cb05ec900643fe7a5f22f7761ab4a@ettus.com>
Reviewed-on: https://chromium-review.googlesource.com/424855
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Martin Roth <79230511ccf79da3f7e9e82dbcc9c1ec2f734fa0@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec