u-boot: Move UBOOT_INITIAL_ENV back to u-boot.inc

Commit cc6c3e31 ("u-boot: Move definitions to common locations") moved
UBOOT_INITIAL_ENV to uboot-config.bbclass, but it should be kept at u-boot.inc
because it encodes ${PN} in it, which should be set by the U-Boot recipe.

Currently, whatever inherits uboot-config bbclass will fill-in its own PN,
which would change the content of UBOOT_INITIAL_ENV per-package.

Cc: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
Cc: Marek Vasut <marex@denx.de>
Fixes: cc6c3e31 ("u-boot: Move definitions to common locations")
Signed-off-by: Fabio Estevam <festevam@denx.de>

Backported from master: 0b0c4b37
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>