Import firmware compiler flags

Import firmware compiler flags

If passed into Makefile, firmware compiler flags (CC and CFLAGS) will
override old values to make sure that the compiled vboot_reference
library has calling convention and alike compatible with the firmware.

CL:6257018 had caused buildbot failure due to combination of two
reasons:

First, the Makefile rule for tlcl_generator that uses HOSTCC, which is
not the cross-compiler passed to Makefile, but wrongly passes CFLAGS,
rather than stuff like HOSTCFLAGS, into HOSTCC.

Second, CL:6257018 changed the behavior of Makefile, and so it did not
override CFLAGS value passed by emerge-${BOARD}, which is incompatible
with HOSTCC.

Thus, this CL preserves the previous the behavior of Makefile that
always overrides CFLAGS value received from outside world.

See also CL:6355014

BUG=None
TEST=See below

1. Run emerge-tegra2_seaboard vboot_reference-firmware successfully, and
check if vboot.a is compiled with CFLAGS from u-boot-cflags.mk

2. Run emerge and emerge-${BOARD} vboot_reference successfully

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=34be827

[Tom Wai-Hong Tam, 2011-01-27 08:15:53]

LGTM