u-boot: relocate SRC_URI_RISCV into recipe

u-boot: relocate SRC_URI_RISCV into recipe

In a custom u-boot recipe that requires u-boot-common.inc, the config
fragments end up in SRC_URI. However, the custom recipe's FILESPATH does
not contain the path to oe-core's u-boot directory (as expected), which
leads to the error below:

ERROR: .../build/../meta-custom/recipes-bsp/u-boot/u-boot-custom_2025.10.bb:
Unable to get checksum for u-boot-custom SRC_URI entry u-boot-riscv-isa_clear.cfg: file could not be found
The following paths were searched:
(...)

Move SRC_URI_RISCV - along with the appends - out of the common
definitions.

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>