u-boot: Relax KCFLAGS warnings for GCC 15

Append -Wno-error flags for int-conversion, implicit function
declarations, and format strings to KCFLAGS.

Tested:
Verified build with GCC 15 and C++23 across gBMC platforms.
- platform11: http://fusion2/3ff2006d-9c03-3f0c-99fd-bcefc6aaca1f
- platform11: http://fusion2/982d88b5-28f9-38ef-a3a2-d824b919c13c
- platform5: http://fusion2/a7e89bcd-86a0-33b2-8f33-2bb24ef3afb5
- platform15: http://fusion2/e56dcb19-6dcd-35a0-b5ee-30740fbf27df
- platform17: http://fusion2/d0e6af61-eb95-3a70-a6a3-d1bdab987ea2
- platform18: http://fusion2/cda6ba3a-58f5-3907-b584-04786b9fe634
- platform25: http://fusion2/714cc620-6f0a-3bfd-90dd-95356b62b54b
Google-Bug-Id: 482144905
Change-Id: I7c0884ce11457bc6f6931486a4bf562f9b117234
Signed-off-by: Willy Tu <wltu@google.com>