env: add spi_flash_read_env function

env: add spi_flash_read_env function

The spi_flash_read_env function is a wrapper over spi_flash_read, which
enables the env to read multiple flash page size from flash until '\0\0'
is read or the end of env partition is reached. Instead of reading the
entire env size. When it reads '\0\0', it stops reading further the env
and assumes that the rest of env is '\0'.

This is an optimization for large environments that contain few bytes
environment variables. In this case it doesn't need to read the entire
environment and only few pages.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>