tools/env: cleanup host build flags

This patch makes tools/env/Makefile more similar to tools/imls:
- define HOSTSRCS and HOSTCPPFLAGS, so that .depend generation works.
- include U-Boot headers using -idirafter to prevent picking up
  u-boot/include/errno.h.
- use HOSTCFLAGS_NOPED (fw_env.c does not conform to -pedantic).

In order to cross-compile tools/env, override the HOSTCC variable
as in this example:

  make tools env HOSTCC=bfin-uclinux-gcc

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>
Tested-by: Detlev Zundel <dzu@denx.de>
Tested-by: Steve Sakoman <steve.sakoman@linaro.org>