am335x_evm: Consolidate eMMC partitions with DFU info

>From DFU_ALT_INFO_EMMC (include/environment/ti/dfu.h) we can see that
rootfs will be flashed to second partition on eMMC. But at the moment we
have only one partition in $partitions environment variable. Let's add
"bootloader" partition prior to "rootfs", so that DFU works correctly.
This also fixes eMMC boot, which looks for rootfs on second partition.

"bootloader" partition start corresponds to "u-boot.img.raw" in DFU
eMMC info, which is 0x300 sector (384 KiB offset from eMMC start).

rootfs start address can be also found from DFU eMMC info.
bootloader-related area is finished at 0x1500 sector (2688 KiB offset
from eMMC start). This should be the start address for rootfs in
$partitions environment variable.

While at it, fix U-Boot environment address to be the same as for
AM57x EVM, so that it doesn't clash with other partitions.

So now eMMC layout looks like this:

    ===============================================================

    0       +------------------------+
            | MBR/GPT header         |   128           -
    128     +------------------------+
            | MLO                    |   256           -
    384     +------------------------+
            | u-boot.img             |   1792          bootloader
    2176    +------------------------+
            | //////// hole //////// |   256           -
    2432    +------------------------+
            | U-Boot environment     |   128           -
    2560    +------------------------+
            | U-Boot environment     |   128           -
            | (redundant)            |
    2688    +------------------------+
            | rootfs                 |   remaining     rootfs
    end     +------------------------+

    ===============================================================

"hole" area can be used further for storing U-Boot environment (like
it's done in AM57x EVM config file) or for increasing u-boot.img area
(in case u-boot.img size increased, e.g. if new dtbs were added).

This commit conforms with Linux partition table from f6d245b8c56c
("arm: am57xx: Fix Linux boot from eMMC") commit, making things in
uniform way.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>