ARMV7: Vexpress: fix build errors

This patch fixes build errors in the vexpress system:
  - Removed sys_proto.h requirement from syslib.c.
  - Switched vexpress to the default armv7 linker script.
  - Renamed TEXT_BASE to CONFIG_SYS_TEXT_BASE.

Signed-off-by: Matt Waddel <matt.waddel@linaro.org>