boot: bootutil: declare fih_panic_loop() as noreturn

Add noreturn attribute the fih_panic_loop() and add a terminal
endless loop in the function to prevent build warnings like
the below one found when build MCUBOOT in TF-M:

.../arm-none-eabi/include/assert.h: In function '__assert_func':
.../trusted-firmware-m/platform/ext/common/tfm_assert.c:22:1: warning: 'noreturn' function does return
   22 | }
      | ^

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>